Edgar Filing: CERUS CORP

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Filed Pursuant to Rule 424(b)(5)
Registration No. 333-67286

PROSPECTUS SUPPLEMENT

(To Prospectus dated December 17, 2001)

6,000,000 Shares

COMMON STOCK

Cerus Corporation is offering 6,000,000 shares of its common stock.

Our common stock is quoted on the Nasdaq National Market under the symbol "CERS." On June 5, 2003, the reported last sale price of our common stock on the Nasdaq National Market was $10.70 per share.

Investing in our common stock involves risks. See "Risk Factors" beginning on page S-5 of this prospectus supplement.

PRICE $9.63 A SHARE

		Price to Public		Underwriting Discounts and Commissions		Proceeds to Cerus Corporation
Per Share		$9.63		$.58		$9.05
Total		$57,780,000		$3,480,000		$54,300,000

We have granted the underwriter the right to purchase up to an additional 900,000 shares of common stock to cover over-allotments.

The Securities and Exchange Commission and state securities regulators have not approved or disapproved these securities, or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

Morgan Stanley & Co. Incorporated expects to deliver the shares to purchasers on June 11, 2003.

MORGAN STANLEY

June 5, 2003

TABLE OF CONTENTS

PROSPECTUS SUPPLEMENT	Page	PROSPECTUS	Page
Prospectus Supplement Summary	S-3	About this Prospectus	1
Risk Factors	S-5	About Cerus	1
Special Note Regarding Forward-		The Securities We May Offer	1
Looking Statements	S-16	Risk Factors	3
Use of Proceeds	S-17	Special Note Regarding Forward-
Dilution	S-17	Looking Statements	13
Underwriter	S-18	Use of Proceeds	13
Description of Common Stock	S-19	Ratio of Earnings to Fixed Charges	13
Legal Matters	S-19	Description of Debt Securities	14
Where You Can Find More Information	S-19	Description of Capital Stock	28
		Plan of Distribution	32
		Legal Matters	34
		Experts	34
		Where You Can Find More Information	34

Unless stated otherwise, references in this prospectus supplement and the accompanying prospectus to "Cerus," "we," "us," or "our" refer to Cerus Corporation, a Delaware corporation.

This document is in two parts. The first part is this prospectus supplement, which describes the terms of the offering of common stock and also adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference into the accompanying prospectus. The second part is the accompanying prospectus, which gives more general information, some of which may not apply to the common stock. To the extent there is a conflict between the information contained in this prospectus supplement, on the one hand, and the information contained in the accompanying prospectus or any document incorporated by reference therein, on the other hand, the information in this prospectus supplement shall control.

You should rely only on the information contained in this prospectus supplement and contained, or incorporated by reference, in the accompanying prospectus. We have not authorized anyone to provide you with information that is different. We are offering to sell, and seeking offers to buy, shares of common stock only in jurisdictions where offers and sales are permitted. The information contained in this prospectus supplement and contained, or incorporated by reference, in the accompanying prospectus is accurate only as of the respective dates thereof, regardless of the time of delivery of this prospectus supplement and the accompanying prospectus, or of any sale of the common stock. It is important for you to read and consider all information contained in this prospectus supplement and the accompanying prospectus, including the documents incorporated by reference therein, in making your investment decision. You should also read and consider the information in the documents we have referred you to in "Where You Can Find More Information" below.

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PROSPECTUS SUPPLEMENT SUMMARY

This summary highlights information contained elsewhere or incorporated by reference in this prospectus supplement and the accompanying prospectus. This summary does not contain all of the information that you should consider before deciding to invest in our common stock. You should read this entire prospectus supplement and the accompanying prospectus carefully including the "Risk Factors" section contained in this prospectus supplement and our financial statements and the related notes and the other documents incorporated by reference in the accompanying prospectus.

We are developing medical systems and therapeutics based on our proprietary Helinx® technology for controlling biological replication. Our most advanced programs are focused on systems to enhance the safety of blood products used for transfusion. The INTERCEPT Blood System, based on our Helinx technology, is designed to inactivate viruses, bacteria, other pathogens and white blood cells. We also are pursuing therapeutic applications of Helinx technology to treat and prevent serious diseases.

We are developing the INTERCEPT Blood System for platelets, plasma and red blood cells with our development and commercialization partner, Baxter Healthcare Corporation. The INTERCEPT Blood System targets and inactivates blood-borne pathogens, such as HIV and hepatitis B and C, as well as harmful white blood cells, while leaving intact the therapeutic properties of the blood components. The INTERCEPT Blood System inactivates a broad array of pathogens and has the potential to reduce the risk of transmission of pathogens for which testing is not completely effective or is not currently performed. We believe that the INTERCEPT Blood System also has the potential to inactivate new pathogens before they are identified and before tests are developed to detect their presence in donated blood. An estimated four million units of platelets, seven million units of plasma and 37 million units of red blood cells are transfused annually in the United States, Western Europe and Japan.

In 2002, the INTERCEPT Blood System for platelets received CE Mark approval and was commercially launched in Europe. The system also received approval for use with buffy coat platelets in Canada in 2002. We and Baxter have submitted regulatory applications seeking marketing approval in Australia and have begun the regulatory submission process to obtain approval in the United States. The companies are in late stage development of the INTERCEPT Blood System for plasma and red blood cells.

We believe that our Helinx technology may have applications beyond inactivating pathogens in blood products, both in modifying T-cells to improve clinical outcomes of cellular therapies and in producing a vaccine for Epstein-Barr Virus (EBV)-associated lymphoma. Our allogeneic cellular immune therapy (ACIT) program, designed to improve the outcome of bone marrow transplantation procedures through use of T-cells treated with the Helinx technology, and EBV cellular vaccine program are in Phase I clinical trials. In addition to plans to pursue the therapeutic potential of our Helinx technology, we also plan to expand our product candidate pipeline by exploring other development areas where we can address large, unmet medical needs.

We are conducting product development and commercialization activities with Baxter pursuant to agreements for the development, manufacturing and marketing of the INTERCEPT Blood System. These agreements provide for Baxter and us to generally share development expenses, for Baxter's exclusive right and responsibility to market the systems worldwide and for us to receive a share of the gross profits from the sale of the systems. We are also collaborating with the U.S. Armed Forces on several initiatives intended to improve the safety and availability of the military's blood supply. We intend to continue to develop our products together with partners that can provide direct funding and manufacturing, marketing and distribution resources and expertise.

We were incorporated in California in 1991 and reincorporated in Delaware in 1996. Our principal executive offices are located at 2411 Stanwell Drive, Concord, California 94520, and our telephone number is (925) 288-6000. Our web site address is www.cerus.com. The information contained on our web site is not

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incorporated by reference into this prospectus. We have included our web site address in this prospectus supplement only as an inactive textual reference and do not intend it to be an active link to our web site.

Helinx™ is a United States Registered trademark of Cerus. INTERCEPT and INTERCEPT Blood are trademarks of Baxter International, Inc. This prospectus supplement also includes trademarks or trade names owned by other parties.

THE OFFERING

Common stock offered by Cerus		6,000,000 shares
Common stock to be outstanding after the offering		22,002,043 shares
Use of proceeds		For research and development activities and continuing clinical trials, general administrative support, capital expenditures, working capital, the repayment of debt, possible acquisitions and general corporate purposes. See "Use of Proceeds" on page S-17.
Nasdaq National Market symbol		CERS

The information above is based on 16,002,043 shares of common stock outstanding as of April 30, 2003. It does not include the following shares of common stock as of April 30, 2003:

•: 3,089,865 shares of common stock issuable upon the exercise of stock options outstanding at a weighted average exercise price of $32.91 per share;
•: 1,633,369 shares of common stock reserved for future awards under our 1999 Equity Incentive Plan;
•: 54,727 shares of common stock reserved for future issuance under our 1998 Non-Officer Stock Option Plan;
•: 59,721 shares of common stock reserved for future issuance under our 1996 Equity Incentive Plan;
•: 66,962 shares of common stock reserved for future issuance under our Employee Stock Purchase Plan; and
•: 332,700 shares of common stock issuable upon conversion of all of our Series B preferred stock.

Unless otherwise indicated, all information in this prospectus supplement assumes no exercise of the underwriter's over-allotment option to purchase up to 900,000 shares of common stock.

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RISK FACTORS

Investing in our common stock involves a high degree of risk. You should carefully consider the following risk factors and all other information contained in this prospectus supplement and the accompanying prospectus and incorporated by reference into the accompanying prospectus before purchasing our common stock. The risks and uncertainties described below are not the only ones facing us. Additional risks and uncertainties that we are unaware of, or that we currently deem immaterial, also may become important factors that affect us. If any of the following risks occur, our business, financial condition or results of operations could be materially and adversely affected. In that case, the trading price of our common stock could decline, and you may lose some or all of your investment.

Risks Related to Our Business

If our pre-clinical and clinical trials are not successful or the data are not considered sufficient by regulatory authorities to grant marketing approval, Baxter and we will be unable to commercialize our products and generate revenue.

Except for the INTERCEPT Blood System for platelets, which is approved for sale in Europe and Canada, we have no products that have received regulatory approval for commercial sale. Our product candidates are in various stages of development, and we face the risks of failure inherent in developing medical devices and biotechnology products based on new technologies. Our products must satisfy rigorous standards of safety and efficacy before the United States Food and Drug Administration and international regulatory authorities can approve them for commercial use. Our INTERCEPT Blood System and stem cell transplantation programs are undergoing clinical testing. We must provide the FDA and foreign regulatory authorities with pre-clinical, clinical and manufacturing data that demonstrate our products are safe, effective and in compliance with government regulations before the products can be approved for commercial sale.

In 2002, the INTERCEPT Blood System for platelets received CE Mark approval in Europe. Our development and marketing partner, Baxter Healthcare Corporation, will need to complete validation studies and obtain reimbursement approvals in some individual European countries to market our products in those countries. In certain countries, including the United Kingdom, France and Germany, the system must be approved for purchase or use by a specific governmental or non-governmental (such as the Paul Ehrlich Institute in Germany) entity or entities in order for it to be adopted by a specific customer. The level of additional product testing varies by country, but could take more than a year to complete after CE Mark approval. We completed our Phase III clinical trial of the INTERCEPT Blood System for platelets in the United States in March 2001 and have submitted data from this trial, along with several other modules of our pre-market approval application, to the FDA. We plan to perform additional analyses of the clinical trial data and conduct an additional clinical trial to provide supplemental data. Data from the additional analyses and supplemental clinical trial will need to be submitted to the FDA before we can complete our regulatory submission. We have completed Phase IIIa and Phase IIIb clinical trials of the INTERCEPT Blood System for plasma in the United States and are conducting a Phase IIIc clinical trial. We are conducting Phase III clinical trials of INTERCEPT red blood cells in the United States. Our allogeneic cellular immune therapy (referred to as ACIT) program, designed to improve the outcome of bone marrow transplantation procedures through use of T-cells treated with the Helinx technology, is in Phase I clinical trials in the United States. Last, our Epstein-Barr virus (referred to as EBV) cellular vaccine program is in a Phase I clinical trial in the United States. We will have to conduct significant additional research and pre-clinical (animal) and clinical (human) testing before we can file additional applications for product approval with the FDA and foreign regulatory authorities. Clinical trials in particular are expensive and have a high risk of failure. In addition, to compete effectively, our products must be easy to use, cost-effective and economical to manufacture on a commercial scale. Any of our product candidates may fail in the testing phase or may not attain market acceptance, which could prevent us from achieving profitability.

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It may take us several years to complete our clinical testing, and failure can occur at any stage of testing. We cannot rely on interim results of trials to predict their final results, and acceptable results in early trials might not be repeated in later trials. Any trial may fail to produce results satisfactory to the FDA or foreign regulatory authorities. Pre-clinical and clinical data can be interpreted in different ways, which could delay, limit or prevent regulatory approval. Negative or inconclusive results from a pre-clinical study or clinical trial or adverse medical events during a clinical trial could cause a pre-clinical study or clinical trial to be repeated, require other studies to be performed or cause a program to be terminated, even if other studies or trials relating to a program are successful.

We may fail to complete our clinical trials on time or be unable to complete the trials at all.

Significant clinical trial delays would impair our ability to commercialize our products and could allow competitors to bring products to market before we do. Some of our clinical trials involve patient groups with rare medical conditions, which has in the past made, and may continue to make, it difficult to identify and enroll a sufficient number of patients to complete the trials on time. Our Phase III clinical trials of the INTERCEPT Blood System for red blood cells involve patient groups that include a significant percentage of children, which has made, and may continue to make, it difficult to obtain consent to enroll these patients in our trials. Other factors, including the unavailability of blood products or delays in the supply of clinical product material, could also delay our clinical trials. Clinical trials of our ACIT and EBV vaccine programs are sponsored by other organizations, which will further reduce our ability to control the progress of these trials. Our product development costs will increase if we have additional delays in testing or approvals.

We are using prototype components in our pre-clinical studies and clinical trials and have not completed the components' commercial design.

If we fail to develop commercial versions of the systems on schedule, our potential revenue would be delayed or diminished and our competitors may be able to bring products to market before we do. The system disposables and instruments we use in our pre-clinical studies and clinical trials are prototypes of those to be used in the final products. As a result, we plan to perform studies, both pre-clinical and clinical, to demonstrate the acceptability of the commercial configuration and the equivalence of the prototype and the commercial design. However, regulatory authorities may require us to perform additional studies, both pre-clinical and clinical, using the commercial versions of the systems, which may increase our expenses and delay the commercialization of our products.

In addition, the design and engineering effort required to complete the final commercial product is substantial and time-consuming. As with any complex development effort, we expect to encounter design, engineering and manufacturing issues. Such issues have previously arisen, sometimes unexpectedly, and solutions to these issues have not always been readily forthcoming. For example, in the system for plasma, fluid leakage was discovered in some components during the scale-up process for commercial manufacturing, resulting in a delay in expected commercialization. The solution to this issue remains under study, and the time required to identify and implement a solution remains uncertain. Additional unforeseen design, engineering and manufacturing issues may arise in the future, which could increase the development cost and delay commercialization of our products.

Because our product candidates have not been manufactured on a commercial scale, we face manufacturing uncertainties that could limit their commercialization. If our third-party manufacturers fail to produce our compounds and other product components satisfactorily and in sufficient quantities, we may incur delays, shortfalls and additional expenses.

Our product candidates, including many of the components, have never been manufactured on a commercial scale. We intend to use third-party manufacturers to produce commercial quantities of the chemical compounds and other components to be used in our products. These compounds and other

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components have never been produced in commercial quantities. We have an agreement with a manufacturer to produce commercial quantities of amotosalen, which is the compound used in our platelet and plasma systems. We currently do not have any other third-party manufacturing agreements in place for commercial production of other compounds or components. Any additional commercial manufacturer will need to develop new methods and processes to manufacture these compounds on a commercial scale and demonstrate to us, the FDA and foreign regulatory authorities that its commercial scale manufacturing processes comply with government regulations. It may be difficult or impossible to economically manufacture our products on a commercial scale.

A limited number of suppliers manufacture our inactivation compounds for our use in product development, including clinical trials. We are pursuing contracts with manufacturers to produce intermediates to our S-303 compound, which is used in our red blood cell system, and to produce S-303 itself. If any of these manufacturers cannot produce our compounds in the required quantities or to the required standards, we may face delays and shortfalls before we are able to identify alternate or additional manufacturers to meet these requirements. Contracts have not yet been signed for the long-term supply in commercial quantities of the compounds used in our red blood cell system. While alternative suppliers for the inactivation compounds exist, any new manufacturer will have to prove both to us and to the FDA and foreign regulatory authorities that its manufacturing process complies with government regulations. Identifying and qualifying such new suppliers could be expensive and time-consuming.

Baxter is responsible for manufacturing and assembling our pathogen inactivation systems. Baxter intends to rely on third parties to manufacture and assemble some of the system components, many of which are customized and have not been manufactured on a commercial scale. Baxter has not produced the pathogen inactivation systems in commercial quantities and may not be able to manufacture and assemble them, or do so economically. In the United States, studies related to the platelet system disposable and compound manufacturing need to be completed and included in FDA submissions before the FDA would consider the applications for approval. Efforts to modify the design for manufacturing of our plasma system continue, and the timing of our regulatory submission for the plasma system is dependent on the successful completion of this design, which is uncertain.

Baxter has advised us that it intends to purchase certain key components of the pathogen inactivation systems from a limited number of suppliers. Contracts for the long-term supply of certain components have not yet been signed. While we believe there are alternative suppliers for these components, it would be expensive and time-consuming to establish additional or replacement suppliers for these components. If Baxter were unable to find adequate suppliers for these components, we may be required to redesign the systems, which could lead to additional testing and clinical trials. If we were required to redesign the products, our development costs would increase, and our programs could be delayed significantly.

We will need to establish a sufficient shelf life for the components of our products before the FDA will approve our products for sale.

Product stability studies to establish the shelf life of our system disposables have not yet demonstrated a sufficient shelf life. Certain platelet and plasma system disposables and packaging are being redesigned, and product stability will need to be validated through additional studies, which are expensive and time consuming. If sufficient shelf life cannot be demonstrated, the products may not achieve customer acceptance and may not receive regulatory approval in the United States.

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Our products may not achieve acceptance in, or be rapidly adopted by, the health care community.

Even if our product candidates receive regulatory approval for commercial sale, physicians, patients and healthcare payors may not believe that the benefits of using our systems justify their additional cost, because the blood supply has become safer or for other reasons. Baxter's ability to successfully commercialize our products will depend in part on the availability of adequate reimbursement for product costs and related treatment of blood components from governmental authorities and private health care insurers (including health maintenance organizations), which are increasingly attempting to contain health care costs by limiting both the extent of coverage and the reimbursement rate for new tests and treatments. In addition, our products do not inactivate all known pathogens, and the inability of our systems to inactivate certain pathogens may inhibit their acceptance. For logistical and financial reasons, the transfusion industry has not always integrated new technologies into their processes, even those with the potential to improve the safety of the blood supply. Our products may require significant changes to our potential customers' space and staffing requirements and require significant capital investment. If our products fail to achieve market acceptance, we may never become profitable.

We will need to develop and test additional configurations of our pathogen inactivation systems to address the entire market.

In the United States, our efforts to develop our systems to inactivate viruses, bacteria and other pathogens in platelets have focused almost entirely on apheresis platelets collected on Baxter's automated collection platform. Apheresis platelets are platelets that are collected from a single donor using an automated collection machine. Currently, we estimate that the majority of platelets used in the United States are collected by apheresis, with the remainder prepared from pooled random donor platelets. Blood centers in the United States preparing random donor platelets may be reluctant to switch to apheresis collection, and the FDA may require us to make our systems to inactivate viruses, bacteria and other pathogens in platelets compatible with random donor platelets. In order to develop a platelet pathogen inactivation system compatible with random donor platelets, we will need to perform additional product development and testing, including clinical trials. These development activities would increase our costs significantly, and may not be successful. In addition, FDA regulations limit the time from pooling to transfusion to four hours to minimize the proliferation of bacterial contamination in the pooled product. As a result, most pooling occurs in hospitals. Our platelet system is designed for use in blood centers, not at hospitals, and is intended to permit storage and transfusion of treated platelets for up to five days after pooling. The FDA's time limit between pooling and transfusion currently precludes the use of our system with pooled random donor platelets. Although our system is designed to reduce the risk of bacterial contamination, we cannot predict whether the FDA would remove this process time constraint to allow our system to be used with pooled random donor platelets, and we have not yet made a formal request for the FDA to do so.

Baxter is one of three primary manufacturers of equipment for the collection of apheresis platelets in the United States. The equipment, design and materials used to collect the platelets vary from manufacturer to manufacturer. We have conducted our pre-clinical and clinical studies in the United States for apheresis platelets collected using only Baxter's equipment and materials. Under an agreement with Haemonetics Corporation, Baxter has agreed to provide Haemonetics with a platelet storage solution proprietary to Cerus and Baxter, with the objective that platelets collected on certain future Haemonetics apheresis collection equipment may be directly treated using our platelet system. Baxter and we also are adapting our platelet system to allow compatibility with other manufacturers' equipment. Such adaptations will require additional product development and testing, including clinical trials. These development activities will increase our costs significantly, and may not be successful. Market acceptance of the platelet system in the United States may be delayed until the system receives regulatory approval for use on such other equipment.

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In Europe, platelets also are typically prepared from several units of whole blood using a semi-automated process known as the buffy coat process. For platelets prepared by the buffy coat process, our platelet system is approved for use only with Baxter's platelet collection and pooling materials. As a result, market acceptance in Europe of our platelet system for platelets prepared by the buffy coat process will depend on market acceptance of Baxter's platelet collection and pooling sets or on our ability to develop products compatible with other manufacturers' platelet collection and pooling sets. Our platelet system is also approved in Europe for use with Baxter's apheresis collection equipment as well as the apheresis collection equipment of two other manufacturers through the use of preparation kits.

In Canada, our platelet system is approved for use with platelets prepared by the buffy coat process. Blood centers in Canada currently use the platelet rich plasma and single donor collection methods, and do not use the buffy coat process. The primary difference between the methods is the centrifugation process for separating the component from whole blood to obtain a therapeutic dose of platelets. Baxter and we intend to apply for the license to use the platelet system in Canada with single-donor platelets. We will not have product sales in Canada unless we apply for and receive approval for our system in Canada for use with single-donor platelets or Canadian blood centers implement the buffy coat method.

Fresh frozen plasma and red blood cells are also collected by different methods and equipment and in different volumes. Our systems for plasma and red blood cells being developed and tested will not be suitable for all methods, equipment and volumes used to collect these blood components. We will need to develop and test additional configurations of these systems in order to address the entire market.

A small number of customers will determine market acceptance of our pathogen inactivation systems.

Even if our products receive regulatory approval to be commercialized and marketed, due to the intense market concentration, failure to properly market, price or sell our products to any of these large customers could significantly diminish potential product revenue. The market for our pathogen inactivation systems is dominated by a small number of blood collection organizations. In the United States, the American Red Cross collects and distributes approximately 50% of the nation's supply of blood and blood components. Other major United States blood collection organizations include the New York Blood Center and United Blood Services, each of which distributes approximately 6% of the nation's supply of blood and blood components. In many countries of Western Europe and in Japan, various national blood transfusion services or Red Cross organizations collect, store and distribute virtually all of their respective nations' blood and blood components supply. In Europe, the largest markets for our products are in Germany, the United Kingdom and France. Decisions on product adoption are centralized in the United Kingdom and France. In Germany, decision on product adoption is expected to be on a blood center-by-blood center basis. We have not received in-country approvals to market our platelet system in these countries. If we do not receive approvals to market our products in these countries, or if the products are not adopted in these countries, our potential product revenue in Europe will be significantly decreased.

We rely heavily on Baxter for development funding, product engineering, manufacturing, marketing and sales.

We have two development and commercialization agreements with Baxter for our systems to inactivate viruses, bacteria and other pathogens in each of the three commonly transfused blood components: platelets, fresh frozen plasma and red blood cells, and we rely on Baxter for significant financial and technical contributions to these programs. Since the beginning of our relationship with Baxter in 1993 through March 31, 2003, we have received $46.7 million in equity investments from Baxter and $25.9 million from Baxter International Inc. and Subsidiaries Pension Trust, a $50.0 million loan from Baxter Capital Corporation and we have recognized $30.0 million in revenue from Baxter. Our ability to develop, manufacture and market these products successfully depends significantly on Baxter's performance under these agreements.

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•: We rely on Baxter for engineering, manufacturing and supplying components of our pathogen inactivation systems. Under the terms of our agreements, Baxter is responsible for manufacturing or supplying the disposable units, such as blood storage containers and related tubing, as well as any device associated with the inactivation processes. If these agreements were terminated or if Baxter otherwise failed to design or deliver an adequate supply of components, we would be required to identify other third-party component manufacturers. We cannot assure you that we would be able to identify such manufacturers on a timely basis or enter into contracts with such manufacturers on reasonable terms, if at all. Any delay in the availability of devices or disposables from Baxter could delay the submission of the INTERCEPT Blood System for regulatory approval or the market introduction and subsequent sales of the systems. Moreover, the inclusion of components manufactured by others could require us to seek new approvals from regulatory authorities, which could result in delays in product delivery. We may not receive any such required regulatory approvals.
•: We rely on Baxter for the marketing, sales and distribution of our pathogen inactivation systems. We currently have a small marketing group that helps support Baxter's marketing organization; however, we do not intend to develop our own independent marketing and sales organization and expect to continue to rely on Baxter to market and sell the INTERCEPT Blood System. If our joint development agreements with Baxter are terminated or if Baxter is unable to market the products successfully, we will be required to find another marketing, sales and distribution partner or develop these capabilities ourselves. We may not be able to find a suitable partner on favorable terms or on a timely basis, if at all. Developing marketing, sales and distribution capabilities ourselves would delay commercialization of our pathogen inactivation systems and increase our costs.
•: We share control over management decisions. Baxter and we share responsibility for managing the development programs for the pathogen inactivation systems. Management decisions are made by a governance committee, which has equal representation from both Baxter and us. Our interests and Baxter's may not always be aligned. Disagreements with Baxter may be time-consuming to resolve and cause significant delays in the development of our products. If we disagree with Baxter on program direction, a neutral party will make the decision. The neutral party may not decide in our best interest. Under the agreements, Baxter may independently develop a pathogen inactivation system for fresh frozen plasma using a pre-existing technology. Such an effort by Baxter could create conflicts in our joint program for the development of a pathogen inactivation system for fresh frozen plasma.
•: Baxter can terminate our agreements or fail to perform. Any development program under the agreements may be terminated by either party, with 90 days' notice in the case of the platelet program, or 270 days' written notice in the case of the plasma or red blood cell programs. If Baxter terminates the agreements or fails to provide adequate funding to support the product development efforts, we will need to obtain additional funding from other sources and will be required to devote additional resources to the development of our products. We cannot assure you that we would be able to find a suitable substitute partner in a timely manner, on reasonable terms or at all. If we fail to find a suitable partner, our research, development or commercialization of certain planned products would be delayed significantly, which would cause us to incur additional expenditures.

Our products are subject to extensive regulation by domestic and foreign governments.

Our products under development, and anticipated future products, are subject to extensive and rigorous regulation by United States local, state and federal regulatory authorities and by foreign regulatory bodies. These regulations are wide-ranging and govern, among other things:

•: product development;
•: product testing;
•: product manufacturing;

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•: product labeling;
•: product storage;
•: product premarket clearance or approval;
•: product sales and distribution;
•: product use standards and documentation;
•: product advertising and promotion; and
•: product reimbursement.

The FDA and other agencies in the United States and in foreign countries impose substantial requirements upon the manufacturing and marketing of products such as those we are developing. The process of obtaining FDA and other required regulatory approvals is long, expensive and uncertain. The time required for regulatory approvals is uncertain, and the process typically takes a number of years, depending on the type, complexity and novelty of the product. We may encounter significant delays or excessive costs in our efforts to secure necessary approvals or licenses, or we may not be successful at all.

Even if our product candidates receive approval for commercial sale, their marketing and manufacturing will be subject to continuing FDA and other regulatory requirements, such as requirements to comply with good manufacturing practices. The failure to comply with these requirements could result in enforcement action, which could harm our business. Later discovery of problems with a product, manufacturer or facility may result in additional restrictions on the product or manufacturer, including withdrawal of the product from the market. Regulatory authorities may also require post-marketing testing, which can involve significant expense. The government may impose new regulations that could further delay or preclude regulatory approval of our potential products. We cannot predict the impact of adverse governmental regulation that might arise from future legislative or administrative action.

Distribution of our products outside the United States also is subject to extensive government regulation. These regulations vary by country, including the requirements for approvals or clearance to market, the time required for regulatory review and the sanctions imposed for violations. In addition to CE Mark approval in Europe, we will need to obtain regulatory approvals in individual European countries to market our products. The level of additional product testing varies by country, but could take up to six months or more to complete after CE Mark approval. Failure to obtain necessary regulatory approvals or any other failure to comply with regulatory requirements could result in reduced revenue and earnings. In some countries, we may also need to obtain government approvals for reimbursement in order for our product to be adopted. Reimbursement levels in some countries are determined by annual budgeting processes which, in addition to affecting product adoption, will affect the price we will be able to charge for our products.

To support our requests for regulatory approval to market our product candidates, we have conducted and intend to conduct various types of studies including:

•: toxicology studies to evaluate product safety;
•: laboratory and animal studies to evaluate product effectiveness;
•: human clinical trials to evaluate the safety, tolerability and effectiveness of treated blood components; and
•: manufacturing and stability studies.

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against the incremental benefits, which may be less compelling in light of improved safety in the blood supply. In addition, our clinical development plan assumes that we will not be required to perform human clinical studies to demonstrate our systems' ability to inactivate pathogens. Although we have discussed this plan with the FDA and other regulatory authorities, they may find it unacceptable at any time and may require human clinical trials to demonstrate efficacy in inactivating pathogens. Trials of this type may be too large and expensive to be practical.

Regulatory agencies may limit the uses, or indications, for which any of our products are approved. For example, we believe that we will be able to claim the inactivation of particular pathogens only to the extent we have laboratory or animal data to support such claims. After regulatory approval for the initial indications, further clinical trials may be necessary to gain approval for the use of the product for additional indications.

In addition to the regulatory requirements applicable to us and to our products, there are regulatory requirements applicable to our prospective customers, the blood centers that process and distribute blood and blood products. Blood centers and others will likely be required to obtain approved license supplements from the FDA before using products processed with our pathogen inactivation systems. This requirement or FDA delays in approving these supplements may deter some blood centers from using our products. Blood centers that do submit supplements may face disapproval or delays in approval that could provide further delay or deter them from using our products. The regulatory impact on potential customers could slow or limit the potential sales of our products.

Customer adoption of our products will be affected by the availability of reimbursement from governments or other third parties.

Sales of our products may be affected by the availability of reimbursement from governments or other third parties, such as insurance companies. It is difficult to predict the reimbursement status of newly approved, novel medical products. In certain foreign markets, governments have issued regulations relating to the pricing and profitability of medical products and medical products companies. There have been proposals in the United States, at both the federal and state government level, to implement such controls. The growth of managed care in the United States has also placed pressure on the pricing of medical products. These pressures can be expected to continue and may limit the prices Baxter and we can obtain for our products.

We have only a limited operating history, and we expect to continue to generate losses.

We may never achieve a profitable level of operations. To date, we have engaged primarily in research and development. Our development and general and administrative expenses have resulted in substantial losses each year since our inception, including net losses of $36.0 million in 2000, $49.4 million in 2001 and $57.2 million in 2002. As of March 31, 2003, we had an accumulated deficit of approximately $247.4 million. Except for our platelet system, which is approved for sale in Europe, all of our products are in the research and development stage, and we have not received significant revenue from product sales. We have received substantially all of our revenue from our agreements with Baxter and other development partners and from federal research grants and cooperative agreements. We will be required to conduct significant research, development, clinical testing and regulatory compliance activities for each of these products. We expect our losses to continue at least until our product candidates are commercialized and achieve significant market acceptance.

If we fail to obtain the capital necessary to fund our future operations, we will not be able to develop product candidates in our pipeline.

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interest income, will support our current and planned operations until at least mid-2004. Our cash, liquidity and capital requirements will depend on many factors, including the development progress and costs of our programs, payments by Baxter and the United States government, costs related to creating, maintaining and defending our intellectual property position, regulatory approval and successful commercialization of our product candidates, competitive developments and regulatory factors.

We expect to require substantial additional funds for our long-term product development, marketing programs and operating expenses. We do not know if we will be able to raise additional funds on acceptable terms. If we are unable to obtain sufficient additional capital, we may need to delay or cease certain development programs. If we raise additional funds by issuing equity securities, our existing stockholders may experience substantial dilution.

If our competitors develop and market products that are more effective than our product candidates, our commercial opportunity will be reduced or eliminated.

We expect our products to encounter significant competition. Our products may compete with other approaches to blood safety and improving the outcome of stem cell transplantation currently in use, as well as with future products developed by biotechnology and pharmaceutical companies, hospital supply companies, national and regional blood centers and governmental organizations and agencies. Our success will depend in part on our ability to respond quickly to medical and technological changes through the development and introduction of new products. Product development is risky and uncertain, and we cannot assure you that we will develop our products successfully. Competitors' products or technologies may make our products obsolete or non-competitive before we are able to generate any significant revenue. Competitors or potential competitors may have substantially greater financial and other resources than we have. They may also have greater experience in pre-clinical testing, human clinical trials and other regulatory approval procedures. Our ability to compete successfully will depend, in part, on our ability to:

•: attract and retain skilled scientific personnel;
•: develop technologically superior products;
•: develop lower cost products;
•: obtain patent or other proprietary protection for our products and technologies;
•: obtain required regulatory approvals for our products;
•: be early entrants to the market; and
•: manufacture, market and sell our products, independently or through collaborations.

Other groups are developing synthetic blood product substitutes and products to stimulate the growth of platelets, and new methods of testing blood for specific pathogens have recently been approved by the FDA, including tests for bacteria. Several companies are developing tests for West Nile Virus in blood products, although none have been approved for sale to date. Development of any of these technologies could impair the potential market for our products.

We may not be able to protect our intellectual property or operate our business without infringing intellectual property rights of others.

Our commercial success will depend, in part, on obtaining and maintaining patent protection on our products and successfully defending our products against third-party challenges. Our technology will be

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protected from unauthorized use by others only to the extent that it is covered by valid and enforceable patents or effectively maintained as trade secrets. As a result, our success depends in part on our ability to:

•: obtain patents;
•: protect trade secrets;
•: operate without infringing upon the proprietary rights of others; and
•: prevent others from infringing on our proprietary rights.

As of March 31, 2003, we owned 65 issued or allowed United States patents and 46 issued or allowed foreign patents. Our patents expire at various dates between 2003 and 2018. In addition, we have 26 pending United States patent applications and have filed 17 corresponding patent applications under the Patent Cooperation Treaty, which are currently pending in Europe, Japan, Australia and Canada, and of which seven are also pending in China and five are also pending in Hong Kong. In addition, we are a licensee under a license agreement with respect to two United States patents covering inventions pertaining to psoralen-based photochemical decontamination treatment of whole blood or blood components and four United States patents relating to vaccines, as well as related foreign patents. We cannot be certain that our patents or patents that we license from others will be enforceable and afford protection against competitors. Our patents or patent applications, if issued, may be challenged, invalidated or circumvented. Our patent rights may not provide us with proprietary protection or competitive advantages against competitors with similar technologies. Others may independently develop technologies similar to ours or independently duplicate our technologies. For example, a patent has recently issued to a third-party covering methods to remove psoralen compounds from blood products. We have reviewed the patent and believe our work predates the invention disclosed in that patent. We are continuing to review that patent and will make a determination as to whether any action is necessary. Due to the extensive time required for development, testing and regulatory review of our potential products, our patents may expire or remain in existence for only a short period following commercialization. This would reduce or eliminate any advantage of the patents.

We cannot be certain that we were the first to make the inventions covered by each of our issued patents or pending patent applications or that we were the first to file patent applications for such inventions. We may need to license the right to use third-party patents and intellectual property to continue development and commercialization of our products. We may not be able to acquire such required licenses on acceptable terms, if at all. If we do not obtain such licenses, we may need to design around other parties' patents, or we may not be able to proceed with the development, manufacture or sale of our products.

We may face litigation to defend against claims of infringement, assert claims of infringement, enforce our patents, protect our trade secrets or know-how or determine the scope and validity of others' proprietary rights. Patent litigation is costly. In addition, we may require interference proceedings declared by the United States Patent and Trademark Office to determine the priority of inventions relating to our patent applications. Litigation or interference proceedings could be expensive and time consuming, and we could be unsuccessful in our efforts to enforce our intellectual property rights.

We may rely, in certain circumstances, on trade secrets to protect our technology. However, trade secrets are difficult to protect. We protect our proprietary technology and processes, in part, by confidentiality agreements with employees and certain contractors. However, these agreements may be breached, we may not have adequate remedies for any breach or our trade secrets may otherwise become known or be independently discovered by competitors. To the extent that our employees, consultants or contractors use intellectual property owned by others, disputes may also arise as to the rights in related or resulting know-how and inventions.

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We may be liable if our products harm people.

We are exposed to potential liability risks inherent in the testing and marketing of medical devices and products. We may be liable if any of our products cause injury, illness or death. We maintain product liability insurance, but do not know whether the insurance will provide adequate coverage against potential liabilities. If we cannot successfully defend ourselves against product liability claims, we may incur substantial liabilities or be required to limit commercialization of our products.

We may be liable if hazardous materials used in the development of our products harm the environment, our employees or other people.

Our research and development activities involve the controlled use of hazardous materials, including certain hazardous chemicals, radioactive materials and infectious pathogens, such as HIV and hepatitis viruses. Although we believe that our safety procedures for handling and disposing of hazardous materials comply with regulatory requirements, we cannot eliminate the risk of accidental contamination or injury. If an accident occurs, we could be held liable for any damages that result.

If we do not generate sufficient cash flow through product sales revenue or by raising additional capital, then we may not be able to meet our debt obligation in 2008.

In January 2003, we received a $50.0 million loan from Baxter Capital Corporation. The interest rate for the loan is 12% per annum. No repayment of principal and interest is due until January 2008. The loan is secured with collateral based on future revenue from sales of the INTERCEPT Blood System for platelets. Our substantial indebtedness will result in a significant amount of interest expense in future periods. Our indebtedness could have significant additional negative consequences, including limiting our ability to obtain additional financing and to plan for, or react to, changes in our business and the industry in which we compete. If we are unable to satisfy our debt obligation, substantial liquidity problems could result, which would negatively impact our future prospects.

Risks Related to Our Common Stock and this Offering

The market price of our stock may be highly volatile.

The market prices for our securities and those of other emerging medical device and biotechnology companies have been, and may continue to be, volatile. For example, during the period from January 1, 2001 to March 31, 2003, the closing sale price of our common stock as quoted on the Nasdaq National Market fluctuated from a low of $5.59 to a high of $75.35. Announcements may have a significant impact on the market price of our common stock. Such announcements may include:

•: biological or medical discoveries;
•: technological innovations or new commercial services by us or our competitors;
•: developments concerning proprietary rights, including patents and litigation matters;
•: regulatory developments in both the United States and foreign countries;
•: public concern as to the safety of new technologies;
•: general market conditions;
•: comments made by analysts, including changes in analysts' estimates of our financial performance; and
•: quarterly fluctuations in our revenue and financial results.

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market fluctuations may adversely affect the market price of our common stock. In the past, following periods of volatility in the market price of a company's stock, securities class action litigation has occurred against the issuing company. Such litigation could result in substantial costs and a diversion of management's attention and resources, which could have a material adverse effect on our revenue and earnings. Any adverse determination in such litigation could also subject us to significant liabilities.

Management will have broad discretion as to the use of the proceeds from this offering, and we may not use the proceeds effectively.

We have not designated the amount of net proceeds we will use for any particular purpose. Accordingly, our management will have broad discretion as to the application of the net proceeds and could use them for purposes other than those contemplated at the time of this offering. Our stockholders may not agree with the manner in which our management chooses to allocate and spend the net proceeds. Moreover, our management may use the net proceeds for corporate purposes that may not increase our profitability or our market value.

You will experience immediate dilution in the book value per share of the common stock you purchase.

Because the price per share of our common stock being offered is substantially higher than the book value per share of our common stock, you will suffer substantial dilution in the net tangible book value of the common stock you purchase in this offering. Based on the public offering price of $9.63 per share, if you purchase shares of common stock in this offering, you will suffer immediate and substantial dilution of $5.39 per share in the net tangible book value of the common stock. See "Dilution" below for a more detailed discussion of the dilution you will incur in this offering.

SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus supplement, the accompanying prospectus and the documents incorporated by reference in the accompanying prospectus contain forward-looking statements within the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934. These statements relate to future events or our future financial performance. In some cases, you can identify forward-looking statements by terminology such as "may," "will," "should," "intend," "expect," "anticipate," "believe," "estimate," "predict," "potential" or "continue" or the negative of such terms or other comparable terminology. These statements are only predictions and involve known and unknown risks, uncertainties and other factors, including the risks outlined under "Risk Factors" beginning on page S-5 of this prospectus supplement and elsewhere in this prospectus supplement, that may cause our or our industry's actual results, levels of activity, performance or achievements to differ from those expressed or implied by such forward-looking statements. Before deciding to purchase our common stock, you should carefully consider the risks described in the "Risk Factors" section of this prospectus supplement, in addition to the other information set forth in this prospectus supplement, the accompanying prospectus and in the documents incorporated by reference in the accompanying prospectus.

Although we believe that the expectations reflected in the forward-looking statements are reasonable, we cannot guarantee future results, levels of activity, performance or achievements. Except as may be required by law, we do not intend to update any of the forward-looking statements for any reason after the date of this prospectus supplement to conform such statement to actual results or if new information becomes available.

S-16

USE OF PROCEEDS

We estimate that the net proceeds we will receive from this offering will be approximately $54.0 million, after deducting the underwriting discount and commissions and estimated offering expenses. If the underwriter's over-allotment option is exercised in full, we estimate the aggregate net proceeds to us will be approximately $62.1 million. We intend to use the net proceeds from this offering for research and development activities and continuing clinical trials, general administrative suport, capital expenditures, working capital and general corporate purposes. We may also use the proceeds to repay a $50.0 million loan outstanding under a revolving credit facility with Baxter Capital Corporation. The interest rate on the loan is 12% per annum, with no repayment of principal or interest due until January 2008. Amounts drawn down from this credit facility are held in investment securities and may be used for research and development. A portion of the proceeds may be used to acquire or invest in complementary businesses products or technologies although we are not currently in negotiations concerning any such acquisition or investments.

We have not determined the amounts we plan to spend on any of the areas listed above or the timing of these expenditures. As a result, our management will have broad discretion to allocate the net proceeds from this offering. Pending application of the net proceeds as described above, we intend to invest the net proceeds of the offering in short-term, investment-grade, interest-bearing securities.

DILUTION

If you purchase our common stock in this offering, your interest will be diluted to the extent of the difference between the public offering price per share and the net tangible book value per share of our common stock after this offering. We calculate net tangible book value per share by dividing the net tangible book value, tangible assets less total liabilities, by the number of outstanding shares of our common stock.

Our net tangible book value at March 31, 2003, was $39.2 million, or $2.45 per share, based on 15,978,519 shares of our common stock outstanding. After giving effect to the sale of 6,000,000 shares of common stock by us at a public offering price of $9.63 per share, less the underwriting discounts and commissions and our estimated offering expenses, our net tangible book value at March 31, 2003, would be $93.2 million, or $4.24 per share. This represents an immediate increase in the net tangible book value of $1.79 per share to existing stockholders and an immediate dilution of $5.39 per share to investors in this offering. The following table illustrates this per share dilution:

Public offering price per share				$	9.63

	Net tangible book value per share as of March 31, 2003	$	2.45
	Increase per share attributable to new investors		1.79

Net tangible book value per share after this offering					4.24

Dilution per share to new investors					5.39

S-17

UNDERWRITER

Under the terms and subject to the conditions set forth in the underwriting agreement dated the date of this prospectus supplement, Morgan Stanley & Co. Incorporated as underwriter has agreed to purchase, and we have agreed to sell, 6,000,000 shares of our common stock.

The underwriter is offering the shares of common stock subject to its acceptance of the shares from us. The underwriting agreement provides that the obligations of the underwriter to pay for and accept delivery of the shares of common stock offered by this prospectus supplement and accompanying prospectus are subject to the approval of certain legal matters by its counsel and to other conditions. The underwriter is obligated to take and pay for all of the shares of common stock offered by this prospectus supplement if any such shares are purchased. However, the underwriter is not required to take or pay for the shares covered by the underwriter's over-allotment described below.

The underwriter initially proposes to offer part of the shares of common stock directly to the public at the public offering price listed on the cover page of this prospectus supplement and part to certain dealers at a price that represents a concession not in excess of $.35 a share under the public offering price. After the initial offering of the shares of common stock, the offering price and other selling terms may from time to time be varied by the underwriter.

We have granted to the underwriter an option, exercisable within 30 days from the date of this prospectus supplement, to purchase up to an aggregate of 900,000 additional shares of common stock at the public offering price set forth on the cover page of this prospectus supplement, less underwriting discounts and commissions. The underwriter may exercise this option solely for the purpose of covering over-allotments, if any, made in connection with the offering of the shares of common stock offered by this prospectus supplement and the accompanying prospectus. If the underwriter's option is exercised in full, the total price to the public would be $66,447,000, the total underwriter's discounts and commissions would be $4,002,000 and total gross proceeds to us would be $62,445,000.

The underwriter has informed us that it does not intend sales to discretionary accounts to exceed five percent of the total number of shares of common stock offered by it.

Our common stock is quoted on The Nasdaq National Market under the symbol "CERS."

The estimated offering expenses payable by us are approximately $300,000, not including the underwriting discounts and commissions, which includes legal, accounting and printing costs and various other fees associated with registering and listing the common stock.

We and our executive officers and directors have agreed that, without the prior written consent of Morgan Stanley & Co. Incorporated, we will not, during the period ending 90 days after the date of this prospectus supplement:

•: offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, lend, or otherwise transfer or dispose of, directly or indirectly, any shares of common stock or any securities convertible into or exercisable or exchangeable for common stock; or
•: enter into any swap or other arrangement that transfers to another, in whole or in part, any of the economic consequences of ownership of common stock,

whether any transaction described above is to be settled by delivery of common stock, or such other securities, in cash or otherwise.

The restrictions described in the immediately preceding paragraph do not apply to:

•: transactions relating to shares of common stock or other securities acquired in open market transactions after the completion of the offering;

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•: sales by our officers and directors pursuant to plans established in accordance with Section 10b5-1 of the Securities Exchange Act of 1934 with a minimum limit order of $30;
•: transfers of shares of common stock or any security convertible into or exercisable or exchangeable for common stock as a bona fide gift or gifts; and
•: transfers of shares of common stock or any security convertible into or exercisable or exchangeable common stock to affiliates (as defined in Rule 405 under the Securities Act of 1933).

provided that in the case of any transfer or distribution, such donee or distributee shall execute and deliver to Morgan Stanley & Co. Incorporated an agreement to be bound by the restrictions set forth above.

In order to facilitate the offering of the common stock, the underwriter may engage in transactions that stabilize, maintain or otherwise affect the price of the common stock. Specifically, the underwriter may sell more shares than it is obligated to purchase under the underwriting agreement, creating a short position. A short sale is covered if the short position is no greater than the number of shares available for purchase by the underwriter under the over-allotment option. The underwriter can close out a covered short sale by exercising the over-allotment option or purchasing shares in the open market. In determining the source of shares to close out a covered short sale, the underwriter will consider, among other things, the open market price of shares compared to the price available under the over-allotment option. The underwriter may also sell shares in excess of the over-allotment option, creating a naked short position. The underwriter must close out any naked short position by purchasing shares in the open market. A naked short position is more likely to be created if the underwriter is concerned that there may be downward pressure on the price of the common stock in the open market after pricing that could adversely affect investors who purchase in the offering. As an additional means of facilitating the offering, the underwriter may bid for, and purchase, shares of common stock in the open market to stabilize the price of the common stock. These activities may raise or maintain the market price of the common stock above independent market levels or prevent or retard a decline in the market price of the common stock. The underwriter is not required to engage in these activities, and may end any of these activities at any time.

We have agreed to indemnify the underwriter and its affiliates against certain liabilities, including liabilities under the Securities Act of 1933.

DESCRIPTION OF COMMON STOCK

Transfer Agent and Registrar

Wells Fargo Shareowner Services is the transfer agent and registrar for our common stock.

LEGAL MATTERS

The validity of the shares of common stock we are offering will be passed upon for us by Cooley Godward LLP of San Francisco, California. Davis Polk & Wardwell of Menlo Park, California, is representing the underwriter.

WHERE YOU CAN FIND MORE INFORMATION

We have filed with the SEC a registration statement (of which this prospectus supplement and accompanying prospectus form a part) on Form S-3 with respect to the common stock being offered by this prospectus supplement. This prospectus supplement and accompanying prospectus do not contain all of the information set forth in the registration statement and the exhibits thereto. For further information with respect to us and the shares of common stock offered hereby, reference is made to the registration statement, including the exhibits thereto. Statements contained in this prospectus supplement as to the contents of any contract or other document referred to herein are not necessarily complete and, where any contract is an exhibit to the registration statement, each statement with respect to the contract is qualified

S-19

in all respects by the provisions of the relevant exhibit, to which reference is hereby made. You may read and copy any document we file at the Public Reference Section of the Securities and Exchange Commission, 450 Fifth Street, N.W., Room 1024, Washington, D.C. 20549. You may call the SEC at 1-800-SEC-0330 for further information about the operation of the public reference rooms.

We are subject to the information and reporting requirements of the Securities Exchange Act of 1934 and, in accordance therewith, file periodic reports, proxy statements and other information with the SEC.

The SEC maintains a website that contains reports, proxy and information statements and other information regarding registrants that file electronically with the SEC. The address of the SEC's website is http://www.sec.gov.

The SEC allows us to incorporate into this prospectus supplement information that we file with the SEC in other documents, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is an important part of this prospectus supplement. Any statement contained in a document which is incorporated by reference is automatically updated and superseded if such information is contained in this prospectus supplement, or information that we later file with the SEC, modifies and replaces such information. We incorporate by reference the following documents we have filed with the SEC:

•: our Annual Report on Form 10-K for the fiscal year ended December 31, 2002 and filed with the SEC on March 28, 2003;
•: our Quarterly Report on Form 10-Q for the quarter ended March 31, 2003 and filed with the SEC on May 15, 2003);
•: our Current Reports on Form 8-K, filed with the SEC on June 5, 2003, June 4, 2003, April 28, 2003 and February 2, 2003;
•: our Definitive Proxy Statement for our 2003 Annual Meeting of Stockholders, filed with the SEC on April 30, 2003; and
•: the description of our common stock set forth in our registration statement on Form 8-A and filed with the SEC on January 8, 1997.

We will furnish without charge to you, on written or oral request, a copy of any or all of the documents incorporated by reference, including exhibits to these documents. You should direct any requests for documents to Cerus Corporation, Attention: Investor Relations Officer, 2411 Stanwell Drive, Concord, California 94520, telephone: (925) 288-6000.

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PROSPECTUS

$300,000,000

CERUS CORPORATION

DEBT SECURITIES
COMMON STOCK

From time to time, we may offer and sell common stock and/or debt securities.

We will describe in one or more prospectus supplements the securities we are offering and selling, as well as the specific terms of the securities. You should read this prospectus and any prospectus supplements carefully before you invest. This prospectus may not be used to offer or sell any securities unless accompanied by a prospectus supplement.

Our common stock is quoted on the Nasdaq National Market under the symbol "CERS." On December 14, 2001, the last reported sale price for our common stock on the Nasdaq National Market was $48.30 per share.

INVESTING IN OUR DEBT SECURITIES OR COMMON STOCK INVOLVES A HIGH DEGREE OF RISK. SEE "RISK FACTORS" BEGINNING ON PAGE 3.

The securities may be sold directly by us to investors, through agents designated from time to time or to or through underwriters or dealers. For additional information on the methods of sale, you should refer to the section entitled "Plan of Distribution." If any underwriters are involved in the sale of any securities with respect to which this prospectus is being delivered, the names of such underwriters and any applicable commissions or discounts will be set forth in a prospectus supplement. The net proceeds we expect to receive from such sale will also be set forth in a prospectus supplement.

NEITHER THE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIES COMMISSION HAS APPROVED OR DISAPPROVED OF THESE SECURITIES OR DETERMINED IF THIS PROSPECTUS IS TRUTHFUL OR COMPLETE. ANY REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.

December 17, 2001

TABLE OF CONTENTS

		Page
ABOUT THIS PROSPECTUS		1
ABOUT CERUS		1
THE SECURITIES WE MAY OFFER		1
RISK FACTORS		3
SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS		13
USE OF PROCEEDS		13
RATIO OF EARNINGS TO FIXED CHARGES		13
DESCRIPTION OF DEBT SECURITIES		14
DESCRIPTION OF CAPITAL STOCK		28
PLAN OF DISTRIBUTION		32
LEGAL MATTERS		34
EXPERTS		34
WHERE YOU CAN FIND MORE INFORMATION		34

This prospectus is part of a registration statement we filed with the Securities and Exchange Commission. You should rely only on the information we have provided or incorporated by reference in this prospectus or any prospectus supplement. We have not authorized anyone to provide you with additional or different information. We are not making an offer of these securities in any jurisdiction where the offer is not permitted. You should assume that the information in this prospectus or any prospectus supplement is accurate only as of the date on the front of the document and that any information we have incorporated by reference is accurate only as of the date of the document incorporated by reference.

ABOUT THIS PROSPECTUS

This prospectus is part of a Registration Statement on Form S-3 that we filed with the Securities and Exchange Commission using a "shelf" registration process. Under this shelf process, we may offer from time to time any combination of securities described in this prospectus in one or more offerings up to a total amount of $300,000,000. This prospectus provides you with a general description of the securities we may offer. Each time we use this prospectus to offer securities, we will provide a prospectus supplement that will contain specific information about the terms of that offering. The prospectus supplements may also add, update or change information contained in this prospectus. You should read both this prospectus and any prospectus supplement together with additional information described below under the heading "Where You Can Find More Information."

ABOUT CERUS

Cerus Corporation is developing medical systems and therapeutics that provide safer and more effective treatment options to patients. Cerus' product candidates are based on its proprietary Helinx technology for controlling biological replication. Cerus' most advanced programs are focused on systems to enhance the safety of the world's blood supply. These INTERCEPT Blood Systems, based on the Helinx technology, are designed to inactivate viruses, bacteria, other such pathogens and harmful white blood cells. Cerus is also pursuing therapeutic applications of the Helinx technology to treat and prevent serious diseases.

Cerus was incorporated in California in 1991 and reincorporated in Delaware in 1996. Our principal executive offices are located at 2411 Stanwell Drive, Concord, California 94520, and our telephone number is (925) 288-6000. In this prospectus, "Cerus," "we," "us" and "our" refer to Cerus Corporation, unless the context otherwise requires.

Helinx™ is a trademark of Cerus Corporation. INTERCEPT Blood System, INTERCEPT Platelet System, INTERCEPT Plasma System and INTERCEPT Red Blood Cell System are trademarks of Baxter International, Inc. This prospectus also includes trademarks or trade names owned by other parties.

THE SECURITIES WE MAY OFFER

We may offer shares of our common stock and one or more series of debt securities with a total value of up to $300,000,000 from time to time under this prospectus at prices and on terms to be determined by market conditions at the time of offering. Each time we offer securities, we will provide a prospectus supplement that will describe the specific amounts, prices and other important terms of the securities, including, to the extent applicable:

•: maturity;
•: redemption terms;
•: interest rate;
•: listing on a securities exchange;
•: sinking fund terms;
•: amount payable at maturity;
•: currency of payments; and
•: conversion or exchange rights.

The prospectus supplement also may add, update or change information contained in this prospectus or in documents we have incorporated by reference. This prospectus may not be used to consummate a sale of securities unless it is accompanied by a prospectus supplement.

We may sell the securities directly to or through agents, underwriters or dealers. We, and our agents or underwriters, reserve the right to accept or reject all or part of any proposed purchase of securities. If we do offer securities through agents or underwriters, we will include in the applicable prospectus supplement:

•: the names of those agents or underwriters;
•: applicable fees, discounts and commissions to be paid to them; and
•: the net proceeds to us.

Common Stock. We may issue shares of our common stock from time to time. Holders of common stock are entitled to one vote per share on all matters submitted to a vote of stockholders. Subject to any preferences of outstanding shares of preferred stock, holders of common stock are entitled to dividends when and if declared by the board of directors.

Debt Securities. We may offer debt securities from time to time, in one or more series, as either senior or subordinated debt or as senior or subordinated convertible debt. The senior debt securities will rank equally with any other unsecured and unsubordinated debt. The subordinated debt securities will be subordinate and junior in right of payment, to the extent and in the manner described in the instrument governing the debt, to all of our senior indebtedness. Convertible debt securities will be convertible into our common stock. Conversion may be mandatory or at the holder's option and would be at specified conversion rates.

The debt securities will be issued under one or more documents called indentures, which are contracts between us and a national banking association, as trustee. In this prospectus, we have summarized certain general features of the debt securities. We urge you, however, to read the prospectus supplements related to the series of debt securities being offered, as well as the complete indentures that contain the terms of the debt securities. Indentures have been filed as exhibits to the registration statement of which this prospectus is a part, and supplemental indentures and forms of debt securities containing the terms of debt securities being offered will be filed as exhibits to the registration statement or will be incorporated by reference from reports we file with the SEC.

RISK FACTORS

You should carefully consider the following risk factors, in addition to other information included or incorporated by reference in this prospectus and any prospectus supplement, before making an investment decision. Cerus' business faces significant risks. If any of the events or circumstances described in the following risks actually occur, our business may suffer, the trading price of our common stock could decline and you may lose all or part of your investment.

Our products are in development; if our pre-clinical and clinical trials are not successful, we will be unable to commercialize our products and generate revenue.

We have no products that have received regulatory approval for commercial sale. Our product candidates are in various stages of development, and we face the risks of failure inherent in developing medical devices and biotechnology products based on new technologies. Our products must satisfy rigorous standards of safety and efficacy before the United States Food and Drug Administration and international regulatory authorities can approve them for commercial use. Our platelet, fresh frozen plasma, red blood cell and stem cell transplantation programs are undergoing clinical testing. We must provide the FDA and foreign regulatory authorities with pre-clinical and clinical data that demonstrate our products are safe and effective before they can be approved for commercial sale.

We have completed our European Phase III (CE Mark) clinical trial of the INTERCEPT Platelet System with random donor platelets, which are platelets prepared from several units of whole blood pooled together in a manual process, and we submitted a CE Mark application for marketing approval in Europe in December 2000. We are conducting a 20-patient ancillary clinical trial in Europe to qualify the commercial configuration of the system. We must complete this trial before the system can receive marketing approval in Europe. We are also conducting a 40-patient ancillary clinical trial in Europe to extend qualification of the system to platelets collected by our development and marketing partner, Baxter Healthcare Corporation's, apheresis collection system, which is a system to collect platelets from a single donor using an automated collection machine. We completed our U.S. Phase III clinical trial of the INTERCEPT Platelet System in March 2001, but we have not yet completed submission of our pre-market approval application with the FDA. We have completed Phase IIIa and Phase IIIb clinical trials of the INTERCEPT Plasma System in the United States and are conducting a Phase IIIc clinical trial. We have completed a Phase Ic clinical trial of the INTERCEPT Red Blood Cell System in the United States and obtained concurrence from the FDA to proceed into Phase III clinical trials. Our allogeneic cellular immune therapy (referred to as ACIT) program, designed to improve the outcome of bone marrow transplantation procedures through use of T-cells treated with the Helinx technology, is in Phase I clinical trials in the United States. Last, our source plasma pathogen inactivation system and Epstein-Barr Virus cellular vaccine program are in pre-clinical development. We will have to conduct significant additional research and pre-clinical (animal) and clinical (human) testing before we can file applications for product approval with the FDA and foreign regulatory agencies. Clinical trials in particular are expensive and have a high risk of failure. In addition, to compete effectively, our products must be easy to use, cost-effective and economical to manufacture on a commercial scale. Any of our product candidates may fail in the testing phase or may not attain market acceptance, which could prevent us from achieving profitability.

It may take us several years to complete our clinical testing, and failure can occur at any stage of testing. We cannot rely on interim results of trials to necessarily predict their final results, and acceptable results in early trials might not be repeated in later trials. Any trial may fail to produce results satisfactory to the FDA or foreign regulatory authorities. Pre-clinical and clinical data can be interpreted in different ways, which could delay, limit or prevent regulatory approval. Negative or inconclusive results from a pre-clinical study or clinical trial or adverse medical events during a clinical trial could cause a pre-clinical study or clinical trial to be repeated or a program to be terminated, even if other studies or trials relating to a program are successful.

We may fail to complete our clinical trials on time or be unable to complete them at all.

Some of our clinical trials involve patient groups with rare medical conditions, which has in the past made, and may continue to make, it difficult to identify and enroll a sufficient number of patients to complete the trials on time. Our product development costs will increase if we have additional delays in testing or approvals. Significant clinical trial delays could allow competitors to bring products to market before we do and impair our ability to commercialize our products.

We are using prototype components in our clinical trials and have not completed their commercial design.

The system disposables and ultraviolet light sources we use in our clinical trials are only prototypes of those to be used in the final products. As a result, we plan to perform studies, both pre-clinical and clinical, to demonstrate the equivalence of the prototype and the commercial design. However, regulatory agencies may require us to perform additional studies, both pre-clinical and clinical, using the commercial versions of the systems, which may increase our expenses and delay the commercialization of our products. If we fail to develop commercial versions of the systems on schedule, our competitors may be able to bring products to market before we do, which would delay or diminish our potential revenue.

Because our product candidates have not been manufactured on a commercial scale, we face manufacturing uncertainties that could limit their commercialization.

Our product candidates, and many of their components, have never been manufactured on a commercial scale. We intend to use third-party manufacturers to produce commercial quantities of the chemical compounds to be used in our products to inactivate viruses, bacteria and other pathogens. These inactivation compounds have never been produced in commercial quantities, and we currently do not have any third-party manufacturing agreements in place for their commercial production. Any commercial manufacturers will need to develop new methods and processes to manufacture these compounds on a commercial scale and demonstrate to us, the FDA and foreign regulatory authorities that their commercial scale manufacturing processes comply with government regulations. It may be difficult or impossible to economically manufacture our products on a commercial scale.

Baxter Healthcare Corporation is responsible for manufacturing and assembling our pathogen inactivation systems. Baxter intends to rely on third parties to manufacture and assemble system components, many of which are customized and have not been manufactured on a commercial scale. Baxter has not produced the pathogen inactivation systems in commercial quantities and may not be able to manufacture and assemble them on an economical basis.

If our third-party manufacturers fail to produce our compounds and other product components satisfactorily and in sufficient quantities, we may incur delays, shortfalls and additional expenses.

A limited number of suppliers manufacture our inactivation compounds for our use in product development, including clinical trials. Of these manufacturers, we currently have contracted with one manufacturer to provide enough S-59, the inactivation compound we use in our platelet and fresh frozen plasma systems, to meet our anticipated clinical trial and product development requirements, and we have contracted with one manufacturer to produce an intermediate compound, S-301, which is used by another manufacturer which is producing S-303, the inactivation compound we use in our red blood cell systems. If any of these manufacturers cannot produce our compounds in the required quantities or to the required standards, we may face delays and shortfalls before we are able to identify alternate or additional manufacturers to meet these requirements. While alternative suppliers for the inactivation compounds exist, any new manufacturer will have to prove both to us and to the FDA and foreign regulatory authorities that its manufacturing process complies with government regulations. Identifying and qualifying such new suppliers could be an expensive and time-consuming process.

Baxter has advised us that it intends to purchase certain key components of the pathogen inactivation systems from a limited number of suppliers. While we believe there are alternative suppliers for these components, it would be expensive and time-consuming to establish additional or replacement suppliers for these components. If Baxter were unable to find adequate suppliers for these components, we may be required to redesign the systems, which could lead to additional testing and clinical trials. If we were required to redesign the products, our development costs would increase, and our programs could be delayed significantly.

Our products may not achieve acceptance in or be rapidly adopted by the health care community.

Even if our product candidates receive regulatory approval for commercial sale, physicians, patients and healthcare payors may not believe that the benefits of using our systems justify their additional cost, because the blood supply has become safer or for other reasons. We believe that our ability to successfully commercialize products will depend in part on the availability of adequate reimbursement for product costs and related treatment of blood components from governmental authorities and private health care insurers (including health maintenance organizations), which are increasingly attempting to contain health care costs by limiting both the extent of coverage and the reimbursement rate for new tests and treatments. In addition, our products may not inactivate all known pathogens, and the inability of our systems to inactivate certain pathogens may inhibit their acceptance. In addition, for logistical and financial reasons, the transfusion industry has not always integrated new technologies into their processes, even those with the potential to improve the safety of the blood supply. If our products fail to achieve market acceptance, we may never become profitable.

We will need to develop and test additional configurations of our platelet pathogen inactivation system to address the entire market.

To date, we have focused almost entirely on developing our systems to inactivate viruses, bacteria and other pathogens in platelets in the United States to treat apheresis platelets collected on Baxter's automated collection platform. Apheresis platelets are platelets that are collected from a single donor using an automated collection machine. Platelets prepared from several units of whole blood pooled together in a manual process are known as random donor platelets. Currently, we estimate that the majority of platelets used in the United States are collected by apheresis, with the remainder prepared from pooled random donor platelets. Blood centers in the United States preparing random donor platelets may be reluctant to switch to apheresis collection, and the FDA may require us to make our systems to inactivate viruses, bacteria and other pathogens in platelets compatible with random donor platelets. In order to develop a platelet pathogen inactivation system compatible with random donor platelets, we will need to perform additional product development and testing, including clinical trials. These development activities would increase our costs significantly, and may not be successful. In addition, FDA regulations limit the time from pooling to transfusion to four hours to minimize the proliferation of bacterial contamination in the pooled product. As a result, most pooling occurs in hospitals. Our platelet system is designed for use in blood centers, not at hospitals, and is intended to permit storage and transfusion of treated platelets for up to five days after pooling. The FDA's time limit between pooling and transfusion currently precludes the use of our system with pooled random donor platelets. Although our system is designed to reduce the risk of bacterial contamination, we cannot predict whether the FDA would remove this process time constraint to allow our system to be used with pooled random donor platelets, and we have not yet made a formal request for them to do so.

Baxter is one of three primary manufacturers of equipment for the collection of apheresis platelets. The equipment, design and materials used to collect the platelets vary from manufacturer to manufacturer. We have conducted our pre-clinical and clinical studies in the United States for apheresis platelets collected using only Baxter's equipment and materials. As a result, market acceptance of our

platelet system for apheresis platelets will depend on market acceptance of Baxter's collection equipment. Blood centers using other equipment may be reluctant to replace their existing equipment, and the regulatory agencies may require us to make our systems compatible with other equipment. If we are required to develop platelet pathogen inactivation systems compatible with other manufacturers' equipment, or if we decide to address this broader market, we will need to perform additional product development and testing, including clinical trials. These development activities would increase our costs significantly, and may not be successful.

In Europe, platelets also are typically prepared from several units of whole blood using a semi-automated process known as the buffy coat process. We are conducting our pre-clinical and clinical studies for platelets prepared by the buffy coat process using only Baxter's platelet collection and pooling materials. As a result, market acceptance in Europe of our platelet system for platelets prepared by the buffy coat process will depend on market acceptance of Baxter's platelet collection and pooling sets or on our ability to develop products compatible with other manufacturers' platelet collection and pooling sets. We are conducting a clinical trial of our pathogen inactivation system for apheresis platelets in Europe using largely Baxter's equipment and materials. As a result, market acceptance of our platelet system for apheresis platelets in Europe will depend on market acceptance of Baxter's collection equipment.

If we receive regulatory approval for our products, a small number of customers will determine market acceptance of our pathogen inactivation systems.

The market for our pathogen inactivation systems is dominated by a small number of blood collection centers. In the United States, the American Red Cross collects and distributes approximately 50% of the nation's supply of blood and blood components. Other major United States blood centers include the New York Blood Center and United Blood Services, each of which distributes approximately 6% of the nation's supply of blood and blood components. In Western Europe and Japan, various national blood transfusion services or Red Cross organizations collect, store and distribute virtually all of their respective nations' blood and blood components supply. Even if our products receive regulatory approval to be commercialized and marketed, due to the intense market concentration, failure to properly market, price or sell our products to any of these large customers could significantly diminish potential product revenue.

We rely heavily on Baxter for development funding, manufacturing, marketing and sales.

We have two development and commercialization agreements with Baxter for our systems to inactivate viruses, bacteria and other pathogens in each of the three commonly transfused blood components: platelets, fresh frozen plasma and red blood cells, and we rely on Baxter for significant financial and technical contributions to these programs. Since the beginning of our relationship with Baxter in 1993 through September 30, 2001, we have received $46.7 million in equity investments from Baxter and $25.9 million from the Baxter International Inc. and Subsidiaries Pension Trust, and we have recognized $24.9 million in revenue from Baxter. Our ability to develop, manufacture and market these products successfully depends significantly on Baxter's performance under these agreements.

•: Baxter can terminate our agreements or fail to perform. Baxter can terminate the agreements without cause under certain circumstances. A development program under the agreements may be terminated by either party on 90 days' notice in the case of the platelet program, or 270 days' written notice in the case of the fresh frozen plasma or red blood cell program. If Baxter terminates the agreements or fails to provide adequate funding to support the product development efforts, we will need to obtain additional funding from other sources and will be required to devote additional resources to the development of our products. We cannot assure you that we would be able to find a suitable substitute partner in a timely manner, on reasonable terms or at all. If we fail to find a suitable partner, our research, development or

commercialization of certain planned products would be delayed significantly which would cause us to incur additional expenditures.

•: We rely on Baxter for manufacturing and supplying components of our pathogen inactivation systems. Under the terms of our agreements, Baxter is responsible for manufacturing or supplying the disposable units, such as blood storage containers and related tubing, as well as any device associated with the inactivation processes. If these agreements were terminated or if Baxter otherwise failed to deliver an adequate supply of components, we would be required to identify other third-party component manufacturers. We cannot assure you that we would be able to identify such manufacturers on a timely basis or enter into contracts with such manufacturers on reasonable terms, if at all. Any delay in the availability of devices or disposables from Baxter could delay the submission of INTERCEPT Blood Systems for regulatory approval or the market introduction and subsequent sales of such systems. Moreover, the inclusion of components manufactured by others could require us to seek new approvals from government regulatory authorities, which could result in delays in product delivery. There can be no assurance that we would receive any such required regulatory approvals.
•: We rely on Baxter for the marketing, sales and distribution of our pathogen inactivation systems. We do not have and currently do not plan to develop our own marketing and sales organization. Instead, we plan to rely on Baxter to market and sell the pathogen inactivation systems. If our joint development agreements with Baxter are terminated or if Baxter is unable to market the products successfully, we will be required to find another marketing, sales and distribution partner or develop these capabilities ourselves. We may not be able to find a suitable partner on favorable terms or on a timely basis, if at all. Developing marketing, sales and distribution capabilities ourselves would delay commercialization of our pathogen inactivation systems and increase our costs.
•: We share control over management decisions. Baxter and we share responsibility for managing the development programs for the pathogen inactivation systems. Management decisions are made by a management board that has equal representation from both Baxter and us. Our interests and Baxter's may not always be aligned. Disagreements with Baxter may be time-consuming to resolve, and cause significant delays in the development of our products. If we disagree with Baxter on program direction, a neutral party will make the decision. The neutral party may not decide in our best interest. Under the agreements, Baxter may independently develop a pathogen inactivation system for fresh frozen plasma using a pre-existing technology. Such an effort by Baxter could create conflicts in our joint program for the development of a pathogen inactivation system for fresh frozen plasma.

Our products are subject to extensive regulation by domestic and foreign governments.

•: product development;
•: product testing;
•: product manufacturing;
•: product labeling;
•: product storage;
•: product premarket clearance or approval;

•: product sales and distribution;
•: product use standards and documentation; and
•: product advertising and promotion.

The FDA and other agencies in the United States and in foreign countries impose substantial requirements upon the manufacturing and marketing of products such as those we are developing. The process of obtaining FDA and other required regulatory approvals is long, expensive and uncertain. The time required for regulatory approvals is uncertain and the process typically takes a number of years, depending on the type, complexity and novelty of the product. We may encounter significant delays or excessive costs in our efforts to secure necessary approvals or licenses, or we may not be successful at all.

Even if our product candidates receive approval for commercial sale, their marketing and manufacturing will be subject to continuing FDA and other regulatory requirements, such as requirements to comply with good manufacturing practices. The failure to comply with these requirements could result in enforcement action, which could harm our business. Later discovery of problems with a product, manufacturer or facility may result in additional restrictions on the product or manufacturer, including withdrawal of the product from the market. Regulatory authorities may also require post-marketing testing, which can involve significant expense. The government may impose new regulations, which could further delay or preclude regulatory approval of our potential products. We cannot predict the impact of adverse governmental regulation, which might arise from future legislative or administrative action.

Distribution of our products outside the United States also is subject to extensive government regulation. These regulations, including the requirements for approvals or clearance to market, the time required for regulatory review and the sanctions imposed for violations, vary by country. Failure to obtain necessary regulatory approvals or any other failure to comply with regulatory requirements could result in reduced revenue and earnings.

To support our requests for regulatory approval to market our product candidates, we intend to conduct various types of studies including:

•: toxicology studies to evaluate product safety;
•: laboratory and animal studies to evaluate product effectiveness; and
•: human clinical trials to evaluate the safety, tolerability and effectiveness of treated blood components.

We have conducted many toxicology studies to demonstrate our product candidates' safety, and we plan to conduct additional toxicology studies throughout the product development process. At any time, the FDA and other regulatory authorities may require further toxicology or other studies to further demonstrate our products' safety, which could delay commercialization. We believe the FDA and other regulatory authorities are likely to weigh the potential risks of using our pathogen inactivation products against the incremental benefits, which may be less compelling in light of improved safety in the blood supply. In addition, our clinical development plan assumes that we will not be required to perform human clinical studies to demonstrate our systems' ability to inactivate pathogens. Although we have discussed this plan with the FDA and other regulatory authorities, they may find it unacceptable at any time and may require human clinical trials to demonstrate efficacy in inactivating pathogens. Trials of this type may be too large and expensive to be practical.

Regulatory agencies may limit the uses, or indications, for which any of our products is approved. For example, we believe that we will be able to claim the inactivation of particular pathogens only to the extent we have laboratory or animal data to support such claims. After regulatory approval for the initial indications, further laboratory or clinical studies may be necessary to gain approval for the use of the product for additional indications.

In addition to the regulatory requirements applicable to us and our products, there are regulatory requirements applicable to our prospective customers, the blood centers that process and distribute blood and blood products. Blood centers and others will likely be required to obtain approved license supplements from the FDA before using products processed with our pathogen inactivation systems. This requirement or FDA delays in approving these supplements may deter some blood centers from using our products. Blood centers that do submit supplements may face disapproval or delays in approval that could provide further delay or deter them from using our products. The regulatory impact on potential customers could slow or limit the potential sales of our products.

We have only a limited operating history, and we expect to continue to generate losses.

We may never achieve a profitable level of operations. To date, we have engaged primarily in research and development. Our development and general and administrative expenses have resulted in substantial losses each year since our inception, including net losses of approximately $29.6 million in 1998, $22.6 million in 1999 and $36.0 million in 2000. As of September 30, 2001, we had an accumulated deficit of approximately $159.6 million. All of our products are in the research and development stage, and we have not received any revenue from product sales. We have received all of our revenue from our agreements with Baxter, Kirin and the Consortium for Plasma Science and from federal research grants and cooperative agreements. We will be required to conduct significant research, development, clinical testing and regulatory compliance activities for each of these products. We expect our losses to continue at least until our product candidates are commercialized and achieve significant market acceptance.

If we fail to obtain the capital necessary to fund our future operations, we will not be able to develop product candidates in our pipeline.

Our product development programs are capital-intensive. We expect to continue to spend substantial funds for our operations for the foreseeable future. We believe that our existing capital resources, together with anticipated payments from Baxter, the Consortium, Kirin and the United States government and projected interest income, will support our current and planned operations for at least the next 24 months. Our cash, liquidity and capital requirements will depend on many factors, including additional research and development needs, product testing results, regulatory requirements, competitive pressures and technological advances and setbacks.

We may require substantial additional funds for our long-term product development, marketing programs and operating expenses. We do not know if we will be able to raise additional funds on acceptable terms. If we raise additional funds by issuing equity securities, our existing stockholders may experience substantial dilution.

If our competitors develop and market products that are more effective than our product candidates, our commercial opportunity will be reduced or eliminated.

through the development and introduction of new products. Product development is risky and uncertain, and we cannot assure you that we will develop our products successfully. Competitors' products or technologies may make our products obsolete or non-competitive before we are able to generate any significant revenue. Many of our competitors or potential competitors have substantially greater financial and other resources than we have. They may also have greater experience in pre-clinical testing, human clinical trials and other regulatory approval procedures. Our ability to compete successfully will depend, in part, on our ability to:

•: attract and retain skilled scientific personnel;
•: develop technologically superior products;
•: develop lower cost products;
•: obtain patent or other proprietary protection for our products and technologies;
•: obtain required regulatory approvals for our products;
•: be early entrants to the market; and
•: manufacture, market and sell our products, independently or through collaborations.

Several companies are developing technologies that are, or in the future may be, the basis for products that will directly compete with or reduce the market for our pathogen inactivation systems. A number of companies are specifically focusing on alternative strategies for pathogen inactivation in various blood components. Precision Pharma Service, Inc. has FDA approval to market solvent-detergent treated fresh frozen plasma in the United States. If the treatment of fresh frozen plasma by solvent-detergent becomes a widespread practice, which has not happened to date, it could impair our ability to market our fresh frozen plasma pathogen inactivation system in the United States. In Europe, several companies, including Grifols, Octapharma AG and Maco Pharma International GmbH, are developing or have developed commercial systems to treat fresh frozen plasma.

Other groups are developing synthetic blood product substitutes and products to stimulate the growth of platelets. Development of any of these technologies could impair the potential market for our products.

We may not be able to protect our intellectual property or operate our business without infringing intellectual property rights of others.

Our commercial success will depend, in part, on obtaining and maintaining patent protection on our products and successfully defending our products against third party challenges. Our technology will be protected from unauthorized use by others only to the extent that it is covered by valid and enforceable patents or effectively maintained as trade secrets. As a result, our success depends in part on our ability to:

•: obtain patents;
•: protect trade secrets;
•: operate without infringing upon the proprietary rights of others; and
•: prevent others from infringing on our proprietary rights.

As of September 30, 2001, we owned 55 issued or allowed United States patents and 55 issued or allowed foreign patents. Our patents expire at various dates between 2003 and 2018. In addition, we have 19 pending United States patent applications and have filed 15 corresponding patent applications under the Patent Cooperation Treaty, which are currently pending in Europe, Japan, Australia and Canada, and of which six are also pending in China. In addition, we are a licensee under a license

agreement with Miles, Inc. and Diamond Scientific Corporation with respect to two United States patents covering inventions pertaining to psoralen-based photochemical decontamination treatment of whole blood or blood components and four United States patents relating to vaccines, as well as related foreign patents. We are also a licensee under a license agreement with Emory University with respect to two United States patents covering inventions related to our ACIT program. The license provides for us to make certain future milestone payments to Emory as well as royalties on any sales of products using the licensed technology. We cannot be certain that our patents or patents that we license from others will be enforceable and afford protection against competitors. Our patents or patent applications, if issued, may be challenged, invalidated or circumvented. Our patent rights may not provide us with proprietary protection or competitive advantages against competitors with similar technologies. Others may independently develop technologies similar to ours or independently duplicate our technologies. For example, a patent has recently issued to a third party covering methods to remove psoralen compounds from blood products. We have reviewed the patent and believe our work predates the invention disclosed in that patent. We are continuing to review that patent and will make a determination as to whether any action is necessary. Due to the extensive time required for development, testing and regulatory review of our potential products, our patents may expire or remain in existence for only a short period following commercialization. This would reduce or eliminate any advantage of the patents.

We cannot be certain that we were the first to make the inventions covered by each of our issued patents or pending patent applications or that we were the first to file patent applications for such inventions. We may need to license the right to use third-party patents and intellectual property to continue development and commercialization of our products. We may not be able to acquire such required licenses on acceptable terms, if at all. If we do not obtain such licenses, we may need to design around other parties' patents or we may not be able to proceed with the development, manufacture or sale of our products.

We may face litigation to defend against claims of infringement, assert claims of infringement, enforce our patents, protect our trade secrets or know-how, or determine the scope and validity of others' proprietary rights. Patent litigation is costly. In addition, we may require interference proceedings declared by the United States Patent and Trademark Office to determine the priority of inventions relating to our patent applications. Litigation or interference proceedings could be expensive and time consuming, and we could be unsuccessful in our efforts to enforce our intellectual property rights.

We may rely, in certain circumstances, on trade secrets to protect our technology. However, trade secrets are difficult to protect. We protect our proprietary technology and processes, in part, by confidentiality agreements with employees and certain contractors. There can be no assurance that these agreements will not be breached, that we will have adequate remedies for any breach, or that our trade secrets will not otherwise become known or be independently discovered by competitors. To the extent that our employees, consultants or contractors use intellectual property owned by others, disputes may also arise as to the rights in related or resulting know-how and inventions.

We may be liable if our products harm people.

We are exposed to potential liability risks inherent in the testing and marketing of medical devices and products. We may be liable if any of our products causes injury, illness or death. We intend to obtain product liability insurance before the commercial introduction of any product, but do not know whether we will be able to obtain and maintain such insurance on acceptable terms. Any insurance we obtain may not provide adequate coverage against potential liabilities. If we cannot successfully defend ourselves against product liability claims, we may incur substantial liabilities or be required to limit commercialization of our products.

We may be liable if hazardous materials used in the development of our products harm the environment, our employees or other people.

Our research and development activities involve the controlled use of hazardous materials, including certain hazardous chemicals, radioactive materials and infectious pathogens such as HIV and hepatitis. Although we believe that our safety procedures for handling and disposing of hazardous materials comply with regulatory requirements, we cannot eliminate the risk of accidental contamination or injury. If an accident occurs, we could be held liable for any damages that result.

The market price of our stock may be highly volatile.

The market prices for our securities and those of other emerging medical device and biotechnology companies have been, and may continue to be, volatile. For example, during the period from January 1, 1999 to September 30, 2001, the closing sale price of our common stock as quoted on the Nasdaq National Market fluctuated from a low of $15.44 to a high of $80.50. Announcements may have a significant impact on the market price of our common stock. Such announcements may include:

•: biological or medical discoveries;
•: technological innovations or new commercial services by us or our competitors;
•: developments concerning proprietary rights, including patents and litigation matters;
•: regulatory developments in both the United States and foreign countries;
•: public concern as to the safety of new technologies;
•: general market conditions;
•: comments made by analysts, including changes in analysts' estimates of our financial performance; and
•: quarterly fluctuations in our revenue and financial results.

The stock market has from time to time experienced extreme price and volume fluctuations, which have particularly affected the market prices for emerging biotechnology and medical device companies, and which have often been unrelated to the operating performance of such companies. These broad market fluctuations may adversely affect the market price of our common stock. In the past, following periods of volatility in the market price of a company's stock, securities class action litigation has occurred against the issuing company. Such litigation could result in substantial costs and a diversion of management's attention and resources, which could have a material adverse effect on our revenue and earnings. Any adverse determination in such litigation could also subject us to significant liabilities.

The securities we are offering may not develop an active public market, which could depress the resale price of the securities.

The securities we are offering, other than our common stock, will be new issues of securities for which there is currently no trading market. We cannot predict whether an active trading market for the securities will develop or be sustained. If an active trading market were to develop, the securities could trade at prices that may be lower than the initial offering price of the securities.

SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

Some of the statements in this prospectus and the documents incorporated by reference are forward-looking statements. These statements are based on our current expectations, assumptions, estimates and projections about our business and our industry, and involve known and unknown risks, uncertainties and other factors that may cause our or our industry's results, levels of activity, performance or achievement to be materially different from any future results, levels of activity, performance or achievements expressed or implied in or contemplated by the forward-looking statements. Words such as "believe," "anticipate," "expect," "intend," "plan," "will," "may," "should," "estimate," "predict," "potential," "continue," or the negative of such terms or other similar expressions, identify forward-looking statements. In addition, any statements that refer to expectations, projections or other characterizations of future events or circumstances are forward-looking statements. Our actual results could differ materially from those anticipated in such forward-looking statements as a result of several factors more fully described under the caption "Risk Factors" and in the documents incorporated by reference. The forward-looking statements made in this prospectus relate only to events as of the date on which the statements are made.

USE OF PROCEEDS

Unless otherwise indicated in the prospectus supplement, we currently intend to use the net proceeds from the sale of securities offered by this prospectus for general corporate purposes, including research and development, capital expenditures and to meet working capital needs. We expect from time to time to evaluate the acquisition of businesses, products and technologies for which a portion of the net proceeds may be used, although we currently are not planning or negotiating any such transactions. Pending such uses, we may invest the net proceeds in interest bearing securities.

RATIO OF EARNINGS TO FIXED CHARGES

The following table sets forth our historical ratio of earnings to fixed charges. Earnings consist of income (loss) from continuing operations before income taxes, extraordinary items, cumulative effect of accounting changes, equity in net losses of affiliates and fixed charges. Fixed charges consist of interest expense and capitalized interest.

	Fiscal Year Ended December 31,
	Fiscal Year Ended December 31,					Nine Months Ended September 30, 2001
	1996	1997	1998	1999	2000	Nine Months Ended September 30, 2001
Ratio of earnings to fixed charges (1)	—	—	—	—	—	—

(1): Earnings for the years ended December 31, 1996, 1997, 1998, 1999 and 2000 and for the nine months ended September 30, 2001 were insufficient to cover fixed charges by $10,207,000, $14,664,000, $29,558,000, $22,628,000, $36,033,000 and $35,766,000, respectively. For this reason, no ratios are provided.

DESCRIPTION OF DEBT SECURITIES

Our debt securities, consisting of notes, debentures or other evidences of indebtedness, may be issued from time to time in one or more series. We may issue the senior debt securities and the subordinated debt securities under separate indentures between us, as issuer, and the trustee or trustees identified in the prospectus supplement. The form for each type of indenture is filed as an exhibit to the registration statement of which this prospectus is a part.

The prospectus supplement will describe the particular terms of any debt securities we may offer. The following is a summary of the material provisions of the debt securities and the indentures. For further information about the indentures and the debt securities, you should read the indentures and the description of the debt securities included in the prospectus supplement.

General

The senior debt securities will constitute our unsecured and unsubordinated obligations and the subordinated debt securities will constitute our unsecured and subordinated obligations. A summary description of the subordination provisions is provided below under the caption "Terms of the Subordinated Debt Securities—Subordination". In general, however, if we declare bankruptcy, the senior debt securities will be paid in full before the subordinated debt securities will receive anything.

You should look in the prospectus supplement for the following terms of the debt securities being offered:

•: the debt securities' designation;
•: the aggregate principal amount of the debt securities;
•: the percentage of their principal amount (the price) at which the debt securities will be issued;
•: the date or dates on which the debt securities will mature and the right, if any, to extend such date or dates;
•: the rate or rates, if any, per year, at which the debt securities will bear interest, or the method of determining such rate or rates;
•: the date or dates from which such interest will accrue, the interest payment dates on which such interest will be payable or the manner of determination of such interest payment dates and the record dates for the determination of holders to whom interest is payable on any interest payment dates;
•: the right, if any, to extend the interest payment periods and the duration of that extension;
•: the names and duties of any co-trustees, depositories, authorizing agents, transfer agents or registrars for any series;
•: information describing any book-entry features;
•: authorized denominations, if other than $1,000 and integral multiples of $1,000;
•: provisions for a sinking fund purchase or other analogous fund, if any;
•: the period or periods, if any, within which, the price or prices of which, and the terms and conditions upon which the debt securities may be redeemed, in whole or in part, at our option or at your option;
•: the form of the debt securities;
•: any provisions for payment of additional amounts for taxes and any provision for redemption, if we must pay such additional amounts in respect of any debt security;

•: the terms and conditions, if any, upon which we may have to repay the debt securities early at your option and the price or prices in the currency or currency unit in which the debt securities are payable;
•: the currency, currencies or currency units for which you may purchase the debt securities and the currency, currencies or currency units in which principal and interest, if any, on the debt securities may be payable;
•: whether and under what circumstances we will pay additional amounts on any debt securities held by a person who is not a United States person for tax purposes and whether we can redeem the debt securities if we have to pay additional amounts;
•: the terms and conditions, if any, pursuant to which the debt securities may be converted or exchanged for the cash value of other securities issued by us or by a third party; and
•: any other terms of the debt securities, including any additional events of default or covenants provided for with respect to the debt securities, and any terms that may be required by or advisable under applicable laws or regulations.

You may present debt securities for exchange and you may present debt securities for transfer in the manner, at the places and subject to the restrictions set forth in the debt securities and the prospectus supplement. We will provide you those services without charge, although you may have to pay any tax or other governmental charge payable in connection with any exchange or transfer, as set forth in the indenture.

Debt securities will bear interest at a fixed rate or a floating rate. Debt securities bearing no interest or interest at a rate that at the time of issuance is below the prevailing market rate may be sold at a discount below their stated principal amount. Special United States federal income tax considerations applicable to any such discounted debt securities or to certain debt securities issued at par which are treated as having been issued at a discount for United States federal income tax purposes will be described in the relevant prospectus supplement.

We may issue debt securities with the principal amount payable on any principal payment date, or the amount of interest payable on any interest payment date, to be determined by reference to one or more currency exchange rates, securities or baskets of securities, commodity prices or indices. You may receive a payment of principal on any principal payment date, or a payment of interest on any interest payment date, that is greater than or less than the amount of principal or interest otherwise payable on such dates, depending upon the value on such dates of the applicable currency, security or basket of securities, commodity or index. Information as to the methods for determining the amount of principal or interest payable on any date, the currencies, securities or baskets of securities, commodities or indices to which the amount payable on such date is linked and certain additional tax considerations will be set forth in the applicable prospectus supplement.

Terms of the Senior Debt Securities

Covenants

Financial Information. We will file with the trustee, within 15 days after we are required to file the same under the Securities Exchange Act of 1934, copies of the annual reports and the information, documents and other reports to be filed pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934. We intend to file all such reports, information and documents with the SEC, whether or not required by Section 13 or 15(d), and will send copies to the trustee within such 15 day period.

Consolidation, Merger and Sale of Assets. We may not consolidate with, merge with or into, or sell, convey, transfer, lease, or otherwise dispose of all or substantially all of our property and assets as an entirety or substantially an entirety in one transaction or a series of related transactions to any person

(other than a consolidation with or merger with or into or a sale, conveyance, transfer, lease or other disposition to a wholly-owned subsidiary with a positive net worth; provided that, in connection with any merger of us and a wholly-owned subsidiary, no consideration other than common stock in the surviving person or our common stock shall be issued or distributed to our stockholders) or permit any person to merge with or into us unless:

•: we are the continuing person or the person formed by such consolidation or into which we are merged or that acquired or leased our property and assets shall be a corporation or limited liability company organized and validly existing under the laws of the United States of America or any jurisdiction thereof and shall expressly assume, by a supplemental indenture, executed and delivered to the trustee, all of our obligations on all of the debt securities and under the indenture;
•: immediately after giving effect to such transaction, no default or event of default shall have occurred and be continuing; and
•: we deliver to the trustee an officers' certificate and opinion of counsel, in each case stating that such consolidation, merger, or transfer and such supplemental indenture complies with this provision and that all conditions precedent provided for in the indenture and the debt securities relating to such transaction have been complied with; provided, however, that the foregoing limitations will not apply if, in the good faith determination of our board of directors, whose determination must be set forth in a board resolution, the principal purpose of such transaction is to change our state of incorporation; and provided further that any such transaction shall not have as one of its purposes the evasion of the foregoing limitations.

If the debt securities are convertible for our other securities or other entities, the person with whom we consolidate, merge or sell all of our property must make provisions for the conversion of the debt securities into securities which the holders of the debt securities would have received if they had converted the debt securities before the consolidation, merger or sale.

Events of Default

An event of default for a series of senior debt securities is defined under the senior indenture as being:

•

our default in the payment of principal or premium on the senior debt securities of such series when due and payable whether at maturity, upon acceleration, redemption, or otherwise;

•

our default in the payment of interest on any senior debt securities of such series when due and payable, if that default continues for a period of 30 days;

•

we default in the performance of or we breach any of our other covenants or agreements in the senior indenture applicable to all the senior debt securities or applicable to senior debt securities of such series and that default or breach continues for a period of 90 consecutive days after we receive written notice from the trustee or from the holders of 25% or more in aggregate principal amount of the senior debt securities of such series then outstanding;

•

a court having jurisdiction enters a decree or order for:

•: relief in respect of us in an involuntary case under any applicable bankruptcy, insolvency, or other similar law now or hereafter in effect;
•: appointment of a receiver, liquidator, assignee, custodian, trustee, sequestrator, or similar official of us or for all or substantially all of our property and assets; or
•: the winding up or liquidation of our affairs,

and in each case, such decree or order shall remain unstayed and in effect for a period of 180 consecutive days; or

•

we:

•: commence a voluntary case under any applicable bankruptcy, insolvency, or other similar law now or hereafter in effect, or consent to the entry of an order for relief in an involuntary case under any such law;
•: consent to the appointment of or taking possession by a receiver, liquidator, assignee, custodian, trustee, sequestrator, or similar official of ours or for all or substantially all of our property and assets; or
•: effect any general assignment for the benefit of creditors.

If an event of default, other than an event of default specified in the last two bullet points above, occurs with respect to an issue of senior debt securities and is continuing under the indenture, then, and in each and every such case, either the trustee or the holders of not less than 25% in aggregate principal amount of such senior debt securities of any affected series then outstanding under the indenture by written notice to us and to the trustee, if such notice is given by such holders, may, and the trustee at the request of such holders shall, declare the principal amount of and accrued interest, if any, on such affected series of senior debt securities to be immediately due and payable. Unless otherwise specified in the prospectus supplement relating to a series of debt securities originally issued at a discount, the amount due upon acceleration shall include only the original issue price of the debt securities, the amount of original issue discount accrued to the date of acceleration and accrued interest, if any.

If the event of default occurs because we defaulted on some of our other indebtedness or because the indebtedness becomes accelerated, the trustee or the holders of at least 25% of the aggregate principal amount of the senior debt securities outstanding under the indenture, voting as one class, can accelerate all of the debt securities outstanding under the indenture. If an event of default specified in the last two bullet points above occurs with respect to us, the principal amount of and accrued interest, if any, on each issue of senior debt securities then outstanding shall be and become immediately due and payable without any notice or other action on the part of the trustee or any holder. Upon certain conditions such declarations may be rescinded and annulled and past defaults may be waived by the holders of a majority in aggregate principal amount of an affected series of senior debt securities that has been accelerated. Furthermore, subject to various provisions in the senior indenture, the holders of at least a majority in aggregate principal amount of all the then outstanding senior debt securities of all affected series, each such series voting as a separate class, by notice to the trustee, may waive an existing default or event of default with respect to such series of senior debt securities and its consequences, except a default in the payment of principal of or interest on such senior debt securities or in respect of a covenant or provision of the indenture which cannot be modified or amended without the consent of the holders of each such senior debt securities. Upon any such waiver, such default shall cease to exist, and any event of default with respect to such senior debt securities shall be deemed to have been cured, for every purpose of the senior indenture; but no such waiver shall extend to any subsequent or other default or event of default or impair any right consequent thereto. For information as to the waiver of defaults, see "Modification and Waiver."

The holders of at least a majority in aggregate principal amount of an affected series of senior debt securities outstanding may direct the time, method and place of conducting any proceeding for any remedy available to the trustee or exercising any trust or power conferred on the trustee with respect to such affected series of senior debt securities. However, the trustee may refuse to follow any direction that conflicts with law or the senior indenture, that may involve the trustee in personal liability, or that the trustee determines in good faith may be unduly prejudicial to the rights of holders

of such issue of senior debt securities not joining in the giving of such direction and may take any other action it deems proper that is not inconsistent with any such direction received from holders of such issue of senior debt securities. A holder may not pursue any remedy with respect to the indenture or any series of senior debt securities unless:

•: the holder gives the trustee written notice of a continuing event of default;
•: the holders of at least 25% in aggregate principal amount of such series of senior debt securities then outstanding make a written request to the trustee to pursue the remedy in respect of such event of default;
•: the requesting holder or holders offer the trustee indemnity satisfactory to the trustee against any costs, liability, or expense;
•: the trustee does not comply with the request within 60 days after receipt of the request and the offer of indemnity; and
•: during such 60-day period, the holders of a majority in aggregate principal amount of such series of senior debt securities do not give the trustee a direction that is inconsistent with the request.

These limitations, however, do not apply to the right of any holder of a debt security to receive payment of the principal of or interest, if any, on such senior debt security, or to bring suit for the enforcement of any such payment, on or after the due date for the senior debt securities, which right shall not be impaired or affected without the consent of the holder.

The senior indenture will require certain of our officers to certify, on or before a date not more than 120 days after the end of each fiscal year, as to their knowledge of our compliance with all conditions and covenants under the indenture, such compliance to be determined without regard to any period of grace or requirement of notice provided under the indenture.

Discharge and Defeasance

The senior indenture provides that, except as otherwise provided in this paragraph, we may discharge our obligations with respect to an issue of senior debt securities and the indenture with respect to such series of senior debt securities:

•

if all senior debt securities of such series previously authenticated and delivered with certain exceptions, have been delivered to the trustee for cancellation and we have paid all sums payable by it under the indenture; or

•

•: the senior debt securities of such series mature within one year or all of them are to be called for redemption within one year under arrangements satisfactory to the trustee for giving the notice of redemption;
•: we irrevocably deposit in trust with the trustee, as trust funds solely for the benefit of the holders of the senior debt securities of such series, for that purpose, money or U.S. government obligations or a combination thereof sufficient (unless such funds consist solely of money, in the opinion of a nationally recognized firm of independent public accountants expressed in a written certification thereof delivered to the trustee), without consideration of any reinvestment and after payment of all federal, state and local taxes or other charges and assessments in respect thereof payable by the trustee, to pay principal of and interest on the senior debt securities of such series to maturity or redemption, as the case may be, and to pay all other sums payable by it under the senior indenture; and

•: we deliver to the trustee an officers' certificate and an opinion of counsel, in each case stating that all conditions precedent provided for in the indenture relating to the satisfaction and discharge of the indenture with respect to the senior debt securities of such series have been complied with.

With respect to the first bullet point, only our obligations to compensate and indemnify the trustee and our right to recover excess money held by the trustee under the indenture shall survive. With respect to the second bullet point, only our obligations with respect to the issue of defeased senior debt securities to execute and deliver such senior debt securities for authentication, to set the terms of such senior debt securities, to maintain an office or agency in respect of such senior debt securities, to have moneys held for payment in trust, to register the transfer or exchange of such senior debt securities, to deliver such senior debt securities for replacement or to be canceled, to compensate and indemnify the trustee and to appoint a successor trustee, and our right to recover excess money held by the trustee shall survive until such senior debt securities are no longer outstanding. Thereafter, only our obligations to compensate and indemnify the trustee, and our right to recover excess money held by the trustee shall survive.

The senior indenture also provides that, except as otherwise provided in this paragraph, we:

•: will be deemed to have paid and will be discharged from any and all obligations in respect of a series of senior debt securities, and the provisions of the senior indenture will no longer be in effect with respect to such senior debt securities ("legal defeasance"); and
•: may omit to comply with any term, provision or condition of the senior indenture described above under "Certain Covenants" and such omission shall be deemed not to be an event of default under the third clause of the first paragraph of "Events of Default" with respect to such series of senior debt securities ("covenant defeasance");

provided that the following conditions shall have been satisfied:

•: we have irrevocably deposited in trust with the trustee as trust funds solely for the benefit of the holders of the senior debt securities of such series, for payment of the principal of and interest on the senior debt securities of such series, money or U.S. government obligations or a combination thereof sufficient (unless such funds consist solely of money, in the opinion of a nationally recognized firm of independent public accountants expressed in a written certification thereof delivered to the trustee) without consideration of any reinvestment and after payment of all federal, state and local taxes or other charges and assessments in respect thereof payable by the trustee, to pay and discharge the principal of and accrued interest on the senior debt securities of such series to maturity or earlier redemption (irrevocably provided for under arrangements satisfactory to the trustee), as the case may be;
•: such deposit will not result in a breach or violation of, or constitute a default under, the indenture or any other material agreement or instrument to which we are a party or by which we are bound;
•: no default or event of default with respect to the senior debt securities of such series shall have occurred and be continuing on the date of such deposit;
•: we shall have delivered to the trustee an opinion of counsel that the holders of the senior debt securities of such series then outstanding will not recognize income, gain or loss for federal income tax purposes as a result of our exercising our option under this provision of the indenture and will be subject to federal income tax on the same amount and in the same manner and at the same times as would have been the case if such deposit and defeasance had not occurred (which opinion, in the case of a legal defeasance, shall be based upon a change in

law) or a ruling directed to the trustee received from or a ruling published by the Internal Revenue Service to the same effect; and

•: we have delivered to the trustee an officers' certificate and an opinion of counsel, in each case stating that all conditions precedent provided for in the indenture relating to the defeasance contemplated of the senior debt securities of such series have been complied with.

Subsequent to legal defeasance under the first bullet point above, our obligations with respect to the issue of defeased senior debt securities to execute and deliver such senior debt securities for authentication, to set the terms of such senior debt securities, to maintain an office or agency in respect of such senior debt securities, to have moneys held for payment in trust, to register the transfer or exchange of such senior debt securities, to deliver such debt securities for replacement or to be canceled, to compensate and indemnify the trustee and to appoint a successor trustee, and its right to recover excess money held by the trustee shall survive until such senior debt securities are no longer outstanding. After such senior debt securities are no longer outstanding, in the case of legal defeasance under the first bullet point above, only our obligations to compensate and indemnify the trustee and our right to recover excess money held by the trustee shall survive.

Modification and Waiver

We and the trustee may amend or supplement the senior indenture or the senior debt securities without notice to or the consent of any holder:

•: to cure any ambiguity, defect, or inconsistency in the senior indenture; provided that such amendments or supplements shall not adversely affect the interests of the holders in any material respect;
•: to comply with the provisions described under "Covenants—Consolidation, Merger and Sale of Assets";
•: to comply with any requirements of the SEC in connection with the qualification of the senior indenture under the Trust Indenture Act;
•: to evidence and provide for the acceptance of appointment hereunder by a successor trustee;
•: to establish the form or forms or terms of the senior debt securities as permitted by the senior indenture;
•: to provide for uncertificated senior debt securities and to make all appropriate changes for such purpose;
•: to make any change that does not adversely affect the rights of any holder;
•: to add to our covenants such new covenants, restrictions, conditions or provisions for the protection of the holders, and to make the occurrence, or the occurrence and continuance, of a default in any such additional covenants, restrictions, conditions or provisions an event of default; or
•: to make any change so long as no senior debt securities are outstanding.

Subject to certain conditions, without prior notice to any holder of an issue of senior debt securities, modifications and amendments of the senior indenture may be made by us and the trustee with the written consent of the holders of a majority in principal amount of such series of senior debt securities, and compliance by us with any provision of the indenture with respect to such series of senior debt securities may be waived by written notice to the trustee by the holders of a majority in principal amount of such series of senior debt securities outstanding; provided, however, that each affected holder must consent to any modification, amendment or waiver that,

•: changes the stated maturity of the principal of, or any installment of interest on, any senior debt securities of such series;
•: reduces the principal amount of, or premium, if any, or interest on, any senior debt securities of such series;
•: changes the place or currency of payment of principal of, or premium, if any, or interest on, any senior debt securities of such series;
•: changes the provisions for calculating the optional redemption price, including the definitions relating thereto;
•: changes the provisions relating to the waiver of past defaults or change or impair the right of holders to receive payment or to institute suit for the enforcement of any payment of any senior debt securities of such series on or after the due date therefor;
•: reduces the above-stated percentage of outstanding senior debt securities of such series the consent of whose holders is necessary to modify or amend or to waive certain provisions of or defaults under the indenture;
•: alters or impairs the right to convert the senior debt security at the rate and upon the terms provided in the indenture;
•: waives a default in the payment of principal of or interest on the senior debt securities;
•: adversely affects the rights of such holder under any mandatory redemption or repurchase provision or any right of redemption or repurchase at the option of such holder; or
•: modifies any of the provisions of this paragraph, except to increase any required percentage or to provide that certain other provisions cannot be modified or waived with the consent of the holder of each senior debt security of such series affected by the modification.

It shall not be necessary for the consent of the holders under this section of the indenture to approve the particular form of any proposed amendment, supplement, or waiver, but it shall be sufficient if such consent approves the substance thereof. After an amendment, supplement, or waiver under this section of the indenture becomes effective, we must give to the holders affected thereby a notice briefly describing the amendment, supplement, or waiver. We will mail supplemental indentures to holders upon request. Any failure by us to mail such notice, or any defect therein, shall not, however, in any way impair or affect the validity of any such supplemental indenture or waiver.

With respect to any issue of senior debt securities, neither we nor any of our subsidiaries will, directly or indirectly, pay or cause to be paid any consideration, whether by way of interest, fee, or otherwise, to any holder of any such senior debt securities for or as an inducement to any consent, waiver, or amendment of any of the terms or provisions of such series of senior debt securities or the indenture with respect to such series of senior debt securities unless such consideration is offered to be paid or agreed to be paid to all holders of such senior debt securities of such series that consent, waive, or agree to amend in the time frame set forth in the solicitation documents relating to such consent, waiver, or agreement.

No Personal Liability of Incorporators, Stockholders, Officers, Directors or Employees

The senior indenture provides that no recourse shall be had under or upon any of our obligations, covenants or agreements in the indenture or any supplemental indenture, or in any of the senior debt securities or because of the creation of any indebtedness represented thereby, against any of our incorporators, stockholders, officers, directors or employees or any of their successor persons under any law, statute or constitutional provision or by the enforcement of any assessment or by any legal or equitable proceeding or otherwise. Each holder, by accepting the senior debt securities, waives and releases all such liability.

Concerning the Trustee

The senior indenture provides that, except during the continuance of a default, the trustee will not be liable, except for the performance of such duties as are specifically set forth in the senior indenture. If an event of default has occurred and is continuing, the trustee will exercise such rights and powers vested in it under the senior indenture and will use the same degree of care and skill in its exercise as a prudent person would exercise under the circumstances in the conduct of such person's own affairs.

Governing Law

The indentures and the debt securities will be governed by, and construed in accordance with, the internal laws of the State of New York.

The Trustees

We may have normal banking relationships with the trustees under the indentures in the ordinary course of business.

Terms of the Subordinated Debt Securities

Other than the terms of the subordinated indenture and subordinated debt securities relating to subordination, or otherwise as described in the prospectus supplement relating to a particular series of subordinated debt securities, the terms of the subordinated indenture and subordinated debt securities are identical, in all material respects, to the terms of the senior indenture and senior debt securities.

Subordination

The payment of the principal of, premium, if any, interest on and all other amounts payable under the subordinated debt securities is subordinated, to the extent provided in the indenture, to the prior payment in full of all senior indebtedness (as defined in the indenture and described below). This subordination will not prevent the occurrence of any event of default. The subordinated debt securities are also structurally subordinated to all indebtedness and other liabilities, including trade payables and lease obligations, if any, of our subsidiaries.

Upon any distribution of our assets upon any dissolution, winding up, bankruptcy, insolvency, liquidation, reorganization, receivership or similar proceeding relating to us or our property, an assignment for the benefit of creditors or any marshaling of our assets or liabilities, the holders of senior indebtedness will be entitled to receive payment in full, in cash or other payment satisfactory to the holders of senior indebtedness, of all obligations due in respect of the senior indebtedness before the holders of the subordinated debt securities will be entitled to receive any payment of the principal, premium, if any, interest on, or any other amounts payable in respect of the subordinated debt securities. Until all obligations with respect to senior indebtedness are paid in full in cash or other payment is made satisfactory to the holders of senior indebtedness, any payment on the subordinated debt securities to which the holders of subordinated debt securities would be entitled shall be made to

the holders of senior indebtedness. By reason of the subordination, in the event of our dissolution, winding up, bankruptcy, insolvency, liquidation, reorganization, receivership or similar proceeding relating to us or our property, an assignment for the benefit of creditors or any marshaling of our assets or liabilities, holders of senior indebtedness may receive more, ratably, and the holders of subordinated debt securities may receive less, ratably, than our other creditors.

In the event of any acceleration of the subordinated debt securities because of an event of default, the holders of any senior indebtedness then outstanding would be entitled to payment in full in cash or other payment satisfactory to the holders of senior indebtedness of all obligations in respect of the senior indebtedness before the holders of the subordinated debt securities would be entitled to receive any payment or distribution. The indenture will require that we promptly notify holders of senior indebtedness if payment of the subordinated debt securities is accelerated because of an event of default.

We also may not make any payment upon or in respect of the subordinated debt securities, including upon redemption, if:

•: a default in the payment of the principal of, premium, if any, interest, rent or other obligations in respect of senior indebtedness occurs and is continuing beyond any applicable period of grace, or payment default, or
•: any other default occurs and is continuing with respect to designated senior indebtedness (as defined in the indenture and described below) that permits holders of the designated senior indebtedness as to which the default relates to accelerate its maturity, and the trustee receives a notice of that default (a "payment blockage notice"), from us or other person permitted to give this notice under the indenture, or non-payment default.

Payments on the subordinated debt securities may and shall be resumed (a) in case of a payment default, upon the date on which the payment default is cured or waived or ceases to exist and (b) in case of a non-payment default, the earlier of the date on which the nonpayment default is cured, waived or ceases to exist or 179 days after the date on which the applicable payment blockage notice is received, if the majority of the designated senior indebtedness has not been accelerated, or in the case of any lease, 179 days after notice is received if we have not received notice that the lessor under such lease has exercised its rights to terminate the lease or require us to make an irrevocable offer to terminate the lease following an event of default under the lease. No new period of payment blockage may be commenced pursuant to a payment blockage notice unless and until 365 days have elapsed since the initial effectiveness of the immediately prior payment blockage notice and all scheduled payments of principal, premium, if any, and interest on the subordinated debt securities that have come due have been paid in full in cash. No non-payment default that existed or was continuing on the date of delivery of any payment blockage notice to the trustee shall be, or shall be made, the basis for a subsequent payment blockage notice.

If, notwithstanding the foregoing, the trustee or any holder of the subordinated debt securities receives any payment or distribution of our assets of any kind in contravention of any of the subordination provisions of the indenture, whether in cash, property or securities, including, without limitation, by way of set-off or otherwise, in respect of the subordinated debt securities before all senior indebtedness is paid in full in cash or other payment satisfactory to holders of senior indebtedness, then that payment or distribution will be held by the recipient in trust for the benefit of holders of senior indebtedness of their representatives to the extent necessary to make payment in full in cash or payment satisfactory to the holders of senior indebtedness of all senior indebtedness remaining unpaid, after giving effect to any concurrent payment or distribution, or provision therefor, to or for the holders of senior indebtedness.

The term "designated senior indebtedness" is defined in the indenture to mean our obligations under any senior indebtedness with respect to which the instrument creating or evidencing the same or the assumption or guarantee thereof (or related agreements or documents to which we are a party) expressly provides that the senior indebtedness shall be "designated senior indebtedness" for purposes of the indenture; provided that the instrument, agreement or other document may place limitations and conditions on the right of that senior indebtedness to exercise the rights of designated senior indebtedness. If any payment made to any holder of any designated senior indebtedness or its representative with respect to such designated senior indebtedness is rescinded or must otherwise be returned by such holder or representative upon the insolvency, bankruptcy or reorganization of us or otherwise, our reinstated indebtedness arising as a result of such rescission or return shall constitute designated senior indebtedness effective as of the date of such rescission or return.

The term "indebtedness" is defined in the indenture to mean, with respect to any person (as defined in the indenture), and without duplication:

(a): all indebtedness, obligations and other liabilities (contingent or otherwise) of that person for borrowed money (including obligations of that person in respect of overdrafts, foreign exchange contracts, currency exchange agreements, interest rate protection agreements, and any loans or advances from banks, whether or not evidenced by notes or similar instruments) or evidenced by bonds, debentures, notes or similar instruments (whether or not the recourse of the lender is to the whole of the assets of that person or to only a portion thereof), other than any account payable or other accrued current liability or obligation incurred in the ordinary course of business in connection with the obtaining of materials or services;
(b): all reimbursement obligations and other liabilities (contingent or otherwise) of that person with respect to letters of credit, bank guaranties or bankers' acceptances;
(c): all obligations and liabilities (contingent or otherwise) in respect of leases of that person required, in conformity with generally accepted accounting principles, to be accounted for as capitalized lease obligations on the balance sheet of that person and all obligations and other liabilities (contingent or otherwise) under any lease or related document (including a purchase agreement) entered into for financing purposes in connection with the lease of real property or improvements which provides that that person is contractually obligated to purchase or cause a third party to purchase the leased property or pay or guarantee a minimum residual value of the leased property to the lessor and the obligations of that person under the lease or related document to purchase or to cause a third party to purchase the leased property;
(d): all obligations of that person (contingent or otherwise) with respect to an interest rate or other swap, cap or collar agreement or other similar instrument or agreement or foreign currency hedge, exchange, purchase or similar instrument or agreement;
(e): all direct or indirect guaranties or similar agreements by that person in respect of, and obligations or liabilities (contingent or otherwise) of that person to purchase or otherwise acquire or otherwise assure a creditor against loss in respect of, indebtedness, obligations or liabilities of another person of the kind described in clauses (a) through (d);
(f): any indebtedness or other obligations described in clauses (a) through (e) secured by any mortgage, pledge, lien or other encumbrance existing on property which is owned or held by that person, regardless of whether the indebtedness or other obligation secured thereby shall have been assumed by that person; and
(g): any and all refinancings, replacements, deferrals, renewals, extensions and refundings of, or amendments, modifications or supplements to, any indebtedness, obligation or liability of the kind described in clauses (a) through (f).

The term "senior indebtedness" is defined in the indenture to mean the principal of, premium, if any, interest (including all interest accruing subsequent to the commencement of any bankruptcy or similar proceeding, whether or not a claim for post-petition interest is allowable as a claim in the proceeding) and rent payable on, or termination payment with respect to, or in connection with, and all fees, costs, expenses and other amounts accrued or due on or in connection with, our indebtedness (as defined), whether outstanding on the date of the indenture or thereafter created, incurred, assumed, guaranteed or in effect guaranteed by us (including all refinancings, replacements, deferrals, renewals, extensions or refundings of, or amendments, modifications or supplements to, the foregoing), unless in the case of any particular indebtedness the instrument creating or evidencing the same or the assumption or guarantee thereof expressly provides that the indebtedness shall not be senior in right of payment to the subordinated debt securities or expressly provides that the indebtedness is pari passu or junior to the subordinated debt securities. The term "senior indebtedness" shall include all "designated senior indebtedness." Notwithstanding the foregoing, the term senior indebtedness shall not include our indebtedness to any of our subsidiaries, a majority of the voting stock of which is owned, directly or indirectly, by us.

As of September 30, 2001, we had approximately $89,000 of indebtedness outstanding that would have constituted senior indebtedness. The indenture will not limit the amount of additional indebtedness, including senior indebtedness, which we can create, incur, assume or guarantee, nor will the indenture limit the amount of indebtedness or other liabilities that any subsidiary can create, incur, assume or guarantee.

We are obligated to pay reasonable compensation to the trustee and to indemnify the trustee against specified losses, liabilities or expenses incurred by it in connection with its duties relating to the notes. The trustee's claims for these payments will generally be senior to those of the holders of the subordinated debt securities in respect of all funds collected or held by the trustee.

Convertible Debt Securities

The terms, if any, on which debt securities being offered may be exchanged for or converted into other debt securities or shares of preferred stock, common stock or our other securities or rights (including rights to receive payments in cash or securities based on the value, rate or price of one or more specified commodities, currencies or indices) or securities of other issuers or any combination of the foregoing will be set forth in the prospectus supplement for the debt securities being offered.

Global Securities

We may issue the debt securities in the form of one or more fully registered global securities that will be deposited with a depositary or with a nominee for a depositary identified in the prospectus supplement relating to such series and registered in the name of the depositary or its nominee. In that case, one or more global securities will be issued in a denomination or aggregate denominations equal to the portion of the aggregate principal or face amount of outstanding registered securities of the series to be represented by such global securities. Unless and until the depositary exchanges a global security in whole for securities in definitive registered form, the global security may not be transferred except as a whole by the depositary to a nominee of the depositary or by a nominee of the depositary to the depositary or another nominee of the depositary or by the depositary or any of its nominees to a successor of the depositary or a nominee of such successor.

The specific terms of the depositary arrangement with respect to any portion of a series of securities to be represented by a global security will be described in the prospectus supplement relating to such series. We anticipate that the following provisions will apply to all depositary arrangements.

Ownership of beneficial interests in a global security will be limited to persons that have accounts with the depositary for such global security known as "participants" or persons that may hold interests

through such participants. Upon the issuance of a global security, the depositary for such global security will credit, on its book-entry registration and transfer system, the participants' accounts with the respective principal or face amounts of the securities represented by such global security beneficially owned by such participants. The accounts to be credited shall be designated by any dealers, underwriters or agents participating in the distribution of such securities. Ownership of beneficial interests in such global security will be shown on, and the transfer of such ownership interests will be effected only through, records maintained by the depositary for such global security (with respect to interests of participants) and on the records of participants (with respect to interests of persons holding through participants). The laws of some states may require that certain purchasers of securities take physical delivery of such securities in definitive form. Such limits and such laws may impair the ability to own, transfer or pledge beneficial interests in global securities.

So long as the depositary for a global security, or its nominee, is the registered owner of such global security, such depositary or such nominee, as the case may be, will be considered the sole owner or holder of the securities represented by such global security for all purposes under the applicable indenture, warrant agreement, purchase contract, declaration, guaranteed trust preferred securities guarantee or unit agreement. Except as set forth below, owners of beneficial interests in a global security will not be entitled to have the securities represented by such global security registered in their names, will not receive or be entitled to receive physical delivery of such securities in definitive form and will not be considered the owners or holders thereof under the applicable indenture, warrant agreement, purchase contract, declaration, guaranteed trust preferred securities guarantee or unit agreement. Accordingly, each person owning a beneficial interest in a global security must rely on the procedures of the depositary for such global security and, if such person is not a participant, on the procedures of the participant through which such person owns its interest, to exercise any rights of a holder under the applicable indenture, warrant agreement, purchase contract, declaration, guaranteed trust preferred securities guarantee or unit agreement. We understand that under existing industry practices, if we request any action of holders or if an owner of a beneficial interest in a global security desires to give or take any action which a holder is entitled to give or take under the applicable indenture, warrant agreement, purchase contract, declaration, guaranteed trust preferred securities guarantee or unit agreement, the depositary for such global security would authorize the participants holding the relevant beneficial interests to give or take such action, and such participants would authorize beneficial owners owning through such participants to give or take such action or would otherwise act upon the instructions of beneficial owners holding through them.

Principal, premium, if any, and interest payments on debt securities, and any payments to holders with respect to warrants, purchase contracts, preferred securities, guaranteed trust preferred securities guarantee or units, represented by a global security registered in the name of a depositary or its nominee will be made to such depositary or its nominee, as the case may be, as the registered owner of such global security. None of us, the trustees, the warrant agents, the unit agents or any of our other agents, agent of the trustees or agent of the warrant agents or unit agents will have any responsibility or liability for any aspect of the records relating to or payments made on account of beneficial ownership interests in such global security or for maintaining, supervising or reviewing any records relating to such beneficial ownership interests.

We expect that the depositary for any securities represented by a global security, upon receipt of any payment of principal, premium, interest or other distribution of underlying securities or commodities to holders in respect of such global security, will immediately credit participants' accounts in amounts proportionate to their respective beneficial interests in such global security as shown on the records of such depositary. We also expect that payments by participants to owners of beneficial interests in such global security held through such participants will be governed by standing customer instructions and customary practices, as is now the case with the securities held for the accounts of

customers in bearer form or registered in "street name," and will be the responsibility of such participants.

If the depositary for any securities represented by a global security is at any time unwilling or unable to continue as depositary or ceases to be a clearing agency registered under the Securities Exchange Act of 1934, and we do not appoint a successor depositary registered as a clearing agency under the Securities Exchange Act of 1934 within 90 days, we will issue such securities in definitive form in exchange for such global security. In addition, we may at any time and in our sole discretion determine not to have any of the securities of a series represented by one or more global securities and, in such event, will issue securities of such series in definitive form in exchange for all of the global security or securities representing such securities. Any securities issued in definitive form in exchange for a global security will be registered in such name or names as the depositary shall instruct the relevant trustee, warrant agent or our other relevant agent. We expect that such instructions will be based upon directions received by the depositary from participants with respect to ownership of beneficial interests in such global security.

DESCRIPTION OF CAPITAL STOCK

Our authorized capital stock consists of 50,000,000 shares of common stock, par value $.001 per share, and 5,000,000 shares of preferred stock, par value $.001 per share.

Common Stock

As of October 31, 2001, there were 15,722,064 shares of common stock issued and outstanding

The holders of common stock are entitled to one vote for each share held of record on all matters submitted to a vote of the stockholders. Subject to preferences that may be applicable to any outstanding shares of the preferred stock, the holders of common stock are entitled to receive ratably such dividends as may be declared by the board of directors out of funds legally available therefor. In the event of a liquidation, dissolution or winding up of our company, holders of the common stock are entitled to share ratably in all assets remaining after payment of liabilities and the liquidation preferences of any outstanding shares of preferred stock. Holders of common stock have no preemptive rights and no right to convert their common stock into any other securities. There are no redemption or sinking fund provisions applicable to the common stock. All outstanding shares of common stock are, and all shares of common stock to be outstanding upon the closing of this offering will be, fully paid and nonassessable.

Preferred Stock

As of October 31, 2001, there were 5,000 shares of Series A preferred stock and 3,327 shares of Series B preferred stock issued and outstanding.

Pursuant to our Restated Certificate of Incorporation, our board of directors has the authority, without further action by the stockholders, to issue up to 5,000,000 shares of preferred stock in one or more series and to fix the designations, powers, preferences, privileges and relative participating, optional or special rights and the qualifications, limitations or restrictions thereof, including dividend rights, conversion rights, voting rights, terms of redemption and liquidation preferences, any or all of which may be greater than the rights of the common stock. The board of directors, without stockholder approval, can issue preferred stock with voting, conversion or other rights that could adversely affect the voting power and other rights of the holders of common stock. Preferred stock could thus be issued quickly with terms calculated to delay or prevent a change in control of our company or make removal of management more difficult. Additionally, the issuance of preferred stock may have the effect of decreasing the market price of the common stock and may adversely affect the voting and other rights of the holders of common stock.

Series A Preferred Stock. The holders of Series A preferred stock have no voting rights, except as required under the General Corporation Law of Delaware, and as follows: Without first obtaining the affirmative vote or written consent of the holders of at least a majority of the outstanding shares of Series A preferred stock, voting as a separate class, we may not effect any merger or consolidation in which Cerus is not the surviving entity, or any merger, consolidation or other transaction in which our common stock becomes no longer publicly traded, unless the surviving entity in such a transaction has provided certain contractual rights for the benefit of the holders of Series A preferred stock. Upon any liquidation, dissolution, or winding up of our company, before any payment or distribution of assets shall be made to the holders of common stock, the holders of Series A preferred stock shall be entitled to be paid out of our assets an amount per share of Series A preferred stock equal to $1,000.00, the original issue price.

We have the right to redeem, at the original issue price, all or a portion of the Series A preferred stock upon the approval to market the platelet pathogen inactivation system by the FDA or the comparable approval in Europe under the Platelet Agreement. We and the holders of Series A

preferred stock may require redemption, at the original issue price, of all of the Series A preferred stock upon the termination for any reason of the Platelet Agreement or upon the cessation for any reason of cooperative development work, as specified in the Platelet Agreement. In addition, immediately prior to consummation of a merger or consolidation in which Cerus is not the surviving entity, or any merger, consolidation or other transaction in which our common stock becomes no longer publicly traded, we have the right to redeem all of the Series A preferred stock then outstanding at the original issue price.

The Series A preferred stock automatically converts, at 120% of the average closing price of the common stock for the 30 trading days prior to and including the trading day immediately prior to the approval to market the platelet pathogen inactivation system by the FDA or the comparable approval in Europe under the Platelet Agreement. If the Platelet Agreement is terminated or cooperative development work under the Platelet Agreement ceases, and a redemption notice has not been sent by us or the holder of the Series A preferred stock, the Series A preferred stock shall automatically convert, at a price equal to the average closing price of the common stock for the 30 trading days commencing with the 15th trading day prior to the date of termination or cessation, on the 15th day following date on which such conversion shall occur shall be the 15th day following the Termination Payment Date, as such term is defined in the Platelet Agreement. However, in the event that the approval of our stockholders is required pursuant to Rule 4460(i) of the Nasdaq Stock Market prior to the issuance of any of the shares of common stock issuable upon conversion of the Series A preferred stock, we must obtain such approval by the applicable conversion date, or, if such approval is not obtained, we must redeem any shares of Series A preferred stock that would be convertible into shares of common stock in excess of the limitation specified in Rule 4460(i).

Neither the Series A preferred stock nor any right to receive redemption payments may be assigned, transferred, hypothecated or otherwise alienated by a Series A preferred stock holder without our prior written consent, except (i) in connection with, and to the transferee of, all or substantially all of the business and assets of such holder, or (ii) to a direct or indirect wholly owned subsidiary of Baxter.

Series B Preferred Stock. The holders of Series B preferred stock have no voting rights, except as required under the General Corporation Law of Delaware, and as follows: Without first obtaining the affirmative vote or written consent of the holders of at least a majority of the outstanding shares of each series of preferred stock that is designated as a sub-series of Series B preferred stock, voting together as a separate class, we may not authorize or issue shares of any class or series of stock, or reclassify any class or series of stock, into shares having preference or priority over the Series B preferred stock as to voting, liquidation preference or conversion rights. Upon any liquidation, dissolution, or winding up of our company, before any payment or distribution of assets shall be made to the holders of common stock, Series A preferred stock or any other class or series of stock ranking junior to the Series B preferred stock with respect to liquidation preference, the holders of Series B preferred stock shall be entitled to be paid out of the assets of the company an amount per share of Series B preferred stock equal to the original issue price.

We will have the right to redeem, at the original issue price, all of the Series B preferred stock at any time. At any time after the one-year anniversary of the date of issuance of the Series B preferred stock, each share of Series B preferred stock may, at the option of the holder, be converted at any time into that number of shares of common stock equal to the original issue price divided by 100. However, in the event that the approval of our stockholders is required pursuant to Rule 4460(i) prior to the issuance of any of the shares of common stock issuable upon conversion of the Series B preferred stock, we must obtain such approval by the conversion date, or, if such approval is not obtained, we must redeem any shares of Series B preferred stock that would be convertible into shares of common stock in excess of the limitation specified in Rule 4460(i).

Neither the Series B preferred stock nor any right to receive redemption payments may be assigned, transferred, hypothecated or otherwise alienated by a Series B preferred stock holder without our prior written consent, except (i) in connection with, and to the transferee of, all or substantially all of the business and assets of such holder, or (ii) to a direct or indirect wholly owned subsidiary of Baxter.

Antitakeover Effects of Provisions of Charter Documents and Delaware Law

Charter Documents. Our Restated Certificate and Bylaws include a number of provisions that may have the effect of deterring hostile takeovers or delaying or preventing changes in control or management of our company. First, our board of directors is classified into three classes of directors. Under Delaware law, directors of a corporation with a classified board may be removed only for cause unless the corporation's certificate of incorporation provides otherwise. Our Restated Certificate does not provide otherwise. In addition, the Restated Certificate provides that all stockholder action must be effected at a duly called meeting of stockholders and not by a consent in writing. Further, our Bylaws limit who may call special meetings of the stockholders. Our Restated Certificate does not include a provision for cumulative voting for directors. Under cumulative voting, a minority stockholder holding a sufficient percentage of a class of shares may be able to ensure the election of one or more directors. Finally, our Bylaws establish procedures, including advance notice procedures, with regard to the nomination of candidates for election as directors and stockholder proposals. These and other provisions of our Restated Certificate and Bylaws and Delaware law could discourage potential acquisition proposals and could delay or prevent a change in control or management of our company.

Delaware Takeover Statute. We are subject to the provisions of Section 203 of the Delaware General Corporation Law. In general, the statute prohibits a publicly held Delaware corporation from engaging in a business combination with an interested stockholder for a period of three years after the date of the transaction in which the person became an interested stockholder, unless the business combination is approved in a prescribed manner. For purposes of Section 203, a business combination includes a merger, asset sale or other transaction resulting in a financial benefit to the interested stockholder, and an interested stockholder is a person who, together with affiliates and associates, owns (or within three years prior, did own) 15% or more of the corporation's voting stock.

Rights Plan

In November 1999, our board of directors adopted a Stockholder Rights Plan, pursuant to which one preferred share purchase right was issued as a dividend for each outstanding share of our common stock. Each purchase right entitles the registered holder to purchase from Cerus one one-hundredth of a share of Series C junior participating preferred stock at a price of $170 per one one-hundredth of a preferred share, subject to adjustment. The purchase rights will become exercisable when a person or group (except with respect to certain Cerus stockholders), acquires 15% or more of our outstanding common stock or ten business days after commencement or announcement of a tender or exchange offer for 15% or more of our outstanding common stock. If a person or group acquires 15% or more of our outstanding common stock, all rightsholders except such buyer will be entitled to acquire Cerus common stock at a discount.

Preferred shares purchasable upon exercise of the purchase rights will not be redeemable. Each preferred share will be entitled to a minimum preferential quarterly dividend payment of $1.00 but will be entitled to an aggregate dividend of 100 times the dividend declared per share of common stock. In the event of liquidation, the holders of the preferred shares would be entitled to receive an aggregate payment equal to 100 times the payment made per share of common stock. Each preferred share will have 100 votes, voting together with the common stock. Finally, in the event of any merger, consolidation or other transaction in which shares of common stock are exchanged, each preferred share will be entitled to receive 100 times the amount of consideration received per share of common

stock. These rights are protected by customary anti-dilution provisions. The preferred shares rank junior to any other series of our preferred stock. These rights will expire on November 3, 2009, unless earlier redeemed or exchanged by Cerus.

Registration Rights

Pursuant to an agreement between us and the holders (or their permitted transferees) of approximately 4.3 million shares of common stock and common stock issuable upon conversion of Series A preferred stock, these holders are entitled to certain rights with respect to the registration of such shares under the Securities Act. If we propose to register our common stock, subject to certain exceptions, under the Securities Act, the holders are entitled to notice of the registration and are entitled to include, at our expense, such shares therein, provided that the managing underwriters have the right to limit the number of such shares included in the registration. In addition, certain of the holders may require us, on no more than two occasions and, on one of such occasions, at our expense, to file a registration statement under the Securities Act with respect to their shares of common stock. Such rights may not be exercised until six months after the closing of this offering. Further, certain holders, at their expense, may require us to register the shares on Form S-3 when such form is available to us, subject to certain conditions and limitations. Such right expires in April 2009.

PLAN OF DISTRIBUTION

We may sell the securities separately or together:

•: through one or more underwriters or dealers in a public offering and sale by them;
•: directly to investors; or
•: through agents.

We may sell the securities from time to time in one or more transactions at a fixed price or prices, which may be changed from time to time:

•: at market prices prevailing at the times of sale;
•: at prices related to such prevailing market prices; or
•: at negotiated prices.

We will set forth in a prospectus supplement the terms of the offering of securities, including:

•: the name or names of any agents or underwriters;
•: the purchase price of the securities being offered and the proceeds we will receive from the sale;
•: any over-allotment options under which underwriters may purchase additional securities from us;
•: any agency fees or underwriting discounts and other items constituting agents' or underwriters' compensation;
•: any initial public offering price;
•: any discounts or concessions allowed or reallowed or paid to dealers; and
•: any securities exchanges on which such securities may be listed.

If we use underwriters for a sale of securities, the underwriters will acquire the securities for their own account. The underwriters may resell the securities in one or more transactions, including negotiated transactions, at a fixed public offering price or at varying prices determined at the time of sale. The obligations of the underwriters to purchase the securities will be subject to the conditions set forth in the applicable underwriting agreement. The underwriters will be obligated to purchase all the securities of the series offered if they purchase any of the securities of that series. We may use underwriters with whom we have a material relationship. We will describe in the prospectus supplement naming the underwriter the nature of any such relationship.

Underwriters, dealers or agents may receive compensation in the form of discounts, concessions or commissions from us or our purchasers (as their agents in connection with the sale of securities). These underwriters, dealers or agents may be considered to be underwriters under the Securities Act. As a result, discounts, commissions or profits on resale received by the underwriters, dealers or agents may be treated as underwriting discounts and commissions. The prospectus supplement will identify any such underwriter, dealer or agent, and describe any compensation received by them from us. Any initial public offering price and any discounts or concessions allowed or reallowed or paid to dealers may be changed from time to time.

We may authorize agents or underwriters to solicit offers by certain types of institutional investors to purchase securities from us at the public offering price set forth in the prospectus supplement pursuant to delayed delivery contracts providing for payment and delivery on a specified date in the future. We will describe the conditions to these contracts and the commissions we must pay for solicitation of these contracts in the prospectus supplement.

Underwriters, dealers and agents may be entitled to indemnification by us against certain civil liabilities, including liabilities under the Securities Act, or to contribution with respect to payments made by the underwriters, dealers or agents, under agreements between us and the underwriters, dealers and agents.

We may grant underwriters who participate in the distribution of securities an option to purchase additional securities to cover over-allotments, if any, in connection with the distribution.

All debt securities will be new issues of securities with no established trading market. Underwriters involved in the public offering and sale of debt securities may make a market in the debt securities. However, they are not obligated to make a market and may discontinue market-making activity at any time. No assurance can be given as to the liquidity of the trading market for any debt securities.

Underwriters or agents and their associates may be customers of, engage in transactions with or perform services for us in the ordinary course of business.

Any underwriter may engage in overallotment, stabilizing transactions, short covering transactions and penalty bids in accordance with Regulation M under the Exchange Act. Overallotment involves sales in excess of the offering size, which create a short position. Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specified maximum. Short covering transactions involve purchases of the securities in the open market after the distribution is completed to cover short positions. Penalty bids permit the underwriters to reclaim a selling concession from a dealer when the securities originally sold by the dealer are purchased in a covering transaction to cover short positions. Those activities may cause the price of the securities to be higher than it would otherwise be. If commenced, the underwriters may discontinue any of the activities at any time.

Any underwriters who are qualified market makers on the Nasdaq National Market may engage in passive market making transactions in the securities on the Nasdaq National Market in accordance with Rule 103 of Regulation M, during the business day prior to the pricing of the offering, before the commencement of offers or sales of the securities. Passive market makers must comply with applicable volume and price limitations and must be identified as passive market makers. In general, a passive market maker must display its bid at a price not in excess of the highest independent bid for such security; if all independent bids are lowered below the passive market maker's bid, however, the passive market maker's bid must then be lowered when certain purchase limits are exceeded.

LEGAL MATTERS

The validity of the issuance of the securities offered hereby will be passed upon by Cooley Godward LLP, Palo Alto, California.

EXERTS

Ernst & Young LLP, independent auditors, have audited our financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2000, as set forth in their report, which is incorporated by reference in this prospectus and elsewhere in the registration statement. Our financial statements are incorporated by reference in reliance on Ernst & Young LLP's report, given on their authority as experts in accounting and auditing.

WHERE YOU CAN FIND MORE INFORMATION

We are a reporting company and file annual, quarterly and current reports, proxy statements and other information with the SEC. We have filed with the SEC a registration statement on Form S-3 under the Securities Act with respect to the shares of common stock and debt securities we are offering under this prospectus. This prospectus does not contain all of the information set forth in the registration statement and the exhibits to the registration statement. For further information with respect to us and the securities we are offering under this prospectus, we refer you to the registration statement and the exhibits and schedules filed as a part of the registration statement. You may read and copy the registration statement, as well as our reports, proxy statements and other information, at the SEC's public reference rooms at Room 1024, 450 Fifth Street, N.W., Washington, D.C. 20549, as well as at the SEC's regional offices at 500 West Madison Street, Suite 1400, Chicago, IL 60661 and at Seven World Trade Center, New York, NY 10048. You can request copies of these documents by writing to the SEC and paying a fee for the copying cost. Please call the SEC at 1-800-SEC-0330 for more information about the operation of the public reference rooms. Our SEC filings are also available at the SEC's web site at "http://www.sec.gov." In addition, you can read and copy our SEC filings at the office of the National Association of Securities Dealers, Inc at 1735 K Street, N.W., Washington, D.C. 20006.

The SEC allows us to "incorporate by reference" the information contained in documents that we file with them, which means that we can disclose important information to you by referring to those documents. The information incorporated by reference is considered to be part of this prospectus. Information in this prospectus supersedes information incorporated by reference that we filed with the SEC prior to the date of this prospectus, while information that we file later with the SEC will automatically update and supersede this information. We incorporate by reference the documents listed below, any filings we will make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934 after the date we filed the registration statement of which this prospectus is a part and before the effective date of the registration statement and any future filings we will make with the SEC under those sections.

The following documents filed with the SEC are incorporated by reference in this prospectus:

1.: Our Annual Report on Form 10-K for the year ended December 31, 2000;
2.: Our Quarterly Reports on Form 10-Q for the quarters ended March 31, 2001, June 30, 2001 and September 30, 2001;
3.: Our Current Reports on Form 8-K, filed with the SEC on May 18, 2001, August 17, 2001 and August 28, 2001; and
4.: The description of our common stock set forth in our registration statement on Form 8-A, filed with the SEC on January 8, 1997.

We will furnish without charge to you, on written or oral request, a copy of any or all of the documents incorporated by reference, including exhibits to these documents. You should direct any requests for documents to Cerus Corporation, Attention: Investor Relations Officer, 2525 Stanwell Drive, Suite 300, Concord, California 94520, telephone: (925) 288-6000.

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TABLE OF CONTENTS
PROSPECTUS SUPPLEMENT SUMMARY
THE OFFERING
RISK FACTORS
SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS
USE OF PROCEEDS
DILUTION
UNDERWRITER
DESCRIPTION OF COMMON STOCK
LEGAL MATTERS
WHERE YOU CAN FIND MORE INFORMATION
TABLE OF CONTENTS
ABOUT THIS PROSPECTUS
ABOUT CERUS
THE SECURITIES WE MAY OFFER
RISK FACTORS
SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS
USE OF PROCEEDS
RATIO OF EARNINGS TO FIXED CHARGES
DESCRIPTION OF DEBT SECURITIES
Global Securities
DESCRIPTION OF CAPITAL STOCK
PLAN OF DISTRIBUTION
LEGAL MATTERS
EXERTS
WHERE YOU CAN FIND MORE INFORMATION