As filed with the Securities and Exchange Commission on April 27, 2001

                                                    Registration No. 333-_______
================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C.  20549
                        ________________________________
                                    FORM S-3
                             REGISTRATION STATEMENT
                                     UNDER
                           THE SECURITIES ACT OF 1933
                        ________________________________
                         ALEXION PHARMACEUTICALS, INC.
             (Exact Name of Registrant as Specified in Its Charter)

                                                                    
                 Delaware                                                             13-3648318
(State or Other Jurisdiction of Incorporation                         (I.R.S. Employer Identification Number)
             or Organization)

                        ________________________________
                               352 Knotter Drive
                              Cheshire, CT  06410
                                 (203) 272-2596
  (Address, Including Zip Code, and Telephone Number, Including Area Code, or
                   Registrant's Principal Executive Offices)
                        ________________________________
                               Leonard Bell, M.D.
                         Alexion Pharmaceuticals, Inc.
                               352 Knotter Drive
                              Cheshire, CT  06410
                                 (203) 272-2596
 (Name, Address, Including Zip Code, and Telephone Number, Including Area Code,
                             of Agent for Service)
                        ________________________________
Copies of all communications, including all communications set to the agent for
                          service, should be sent to:

                            Merrill M. Kraines, Esq.
                           Lawrence A. Spector, Esq.
                          Fulbright & Jaworski L.L.P.
                                666 Fifth Avenue
                         New York, New York  10103-3198
                        ________________________________

     Approximate date of commencement of proposed sale to the public:  From time
to time after the effective date of this Registration Statement.  If the only
securities being registered on this Form are being offered pursuant to dividend
or interest reinvestment plan, please check the following box: [ ]

     If any of the securities being registered on this Form are to be offered on
a delayed or continuous basis pursuant to Rule 415 under the Securities Act of
1933, as amended, other than securities offered only in connection with dividend
or interest reinvestment plans, check the following box. [X]

     If this Form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, please check the following box
and list the Securities Act registration statement number of the earlier
effective registration statement for the same offering. [ ]

     If this Form is a post-effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering. [ ]

     If delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box. [ ]

                        CALCULATION OF REGISTRATION FEE



Title Of Shares To Be       Amount To Be     Proposed Maximum Aggregate      Proposed Maximum Aggregate   Amount Of Registration Fee
       Registered             Registered         Price Per Unit (1)                Offering Price
------------------------------------------------------------------------------------------------------------------------------------
                                                                                              
Common Stock, $.0001 par       193,394               $20.68                           $3,999,388                      $1,000
value per share
====================================================================================================================================


(1)  The price is estimated in accordance with Rule 457(c) under the Securities
Act of 1933, as amended, solely for the purpose of calculating the registration
fee and is $20.68, the average of the high and low prices of the common stock of
Alexion Pharmaceuticals, Inc. as reported on The Nasdaq Stock Market on
April 25, 2001.

     This registrant hereby amends this registration statement on such date or
dates as may be necessary to delay its effective date until the registrant shall
file a further amendment which specifically states that this registration
statement shall thereafter become effective in accordance with Section 8(a) of
the Securities Act of 1933 or until the registration statement shall become
effective on such date as the Commission, acting pursuant to said Section 8(a),
may determine.

     The information in this prospectus is not complete and may be changed.  A
registration statement relating to these securities has been filed with the
Securities and Exchange Commission.  These securities may not be sold nor offers
to buy be accepted before the registration statement becomes effective.  This
prospectus is not an offer to sell securities and is not soliciting an offer to
buy these securities in any state where the offer or sale is not valid.


                  Subject to Completion, Dated April 27, 2001

                                 [ALEXION LOGO]

                         187,114 Shares of Common Stock

                           _________________________

     Alexion Pharmaceuticals, Inc.'s common stock trades on the Nasdaq National
Market under the ticker symbol "ALXN."  On April 25, 2001, the closing sale
price of the common stock was $20.10.

                           _________________________

     The stockholders of Alexion Pharmaceuticals, Inc. listed in this prospectus
are offering and selling an aggregate of 187,114 shares of our common stock
under this prospectus.  We issued these selling stockholders this common stock
on September 23, 2000 in connection with our acquisition of Prolifaron, Inc.,
which was owned by these stockholders.

                           _________________________

     The selling stockholders (and their donees and pledgees) may offer their
Alexion common stock through public or private transactions, on or off the
United States exchanges, at prevailing market prices, or at privately negotiated
prices.

                           _________________________

     SEE "RISK FACTORS" BEGINNING ON PAGE 3 FOR A DISCUSSION OF CERTAIN FACTORS
THAT YOU SHOULD CONSIDER BEFORE YOU INVEST IN THE SHARES BEING SOLD WITH THIS
PROSPECTUS.

                           _________________________

     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.

                           _________________________

                 The date of this Prospectus is ________, 2001.

The information in this prospectus is not complete and my be changed.  A
registration statement relating to these securities has been filed with the
Securities and Exchange Commission.  These securities may not be sold nor may
offers to buy be accepted before the registration statement becomes effective.
This prospectus is not an offer to sell securities and is not soliciting an
offer to buy these securities in any state where the offer or sale is not
permitted.


                               TABLE OF CONTENTS


                                                                         Page
                                                                         ----

Our Company................................................................ 1
Risk Factors............................................................... 3
Special Note Regarding Forward-Looking Statements.......................... 9
Use of Proceeds............................................................ 9
Dividend Policy............................................................ 9
Selling Stockholders........................................................9
Plan of Distribution.......................................................11
Legal Matters..............................................................11
Experts....................................................................11
Where You Can Find More Information........................................12

                         ______________________________


                                  OUR COMPANY

     This summary provides an overview of selected information and does not
contain all the information you should consider. You should read the entire
prospectus, including the section entitled "Risk Factors," carefully before
making an investment decision.

     We are engaged in the development of therapeutic products aimed at treating
patients with a wide array of severe disease states, including cardiovascular
and autoimmune disorders, inflammation and cancer.  Since our inception in
January 1992, we have devoted substantially all of our resources to drug
discovery, research, product and clinical development.  Since mid-1996, we have
focused our resources increasingly on clinical manufacturing and clinical
development.  We are currently examining our two lead genetically altered or
humanized antibody product candidates in eight different clinical development
programs.

     Antibodies are proteins that bind to specific targets and are used by the
immune system to protect the body.  We have proprietary rights to humanized and
human antibodies that can potentially be used in treatments for heart disease,
diseases of the immune system, inflammation and cancer.  In September 2000, we
augmented our antibody product discovery and development program through the
acquisition of Prolifaron, Inc.  Prolifaron was a biopharmaceutical company with
rights to a portfolio of potential antibody product candidates and with know-how
and proprietary technology for discovering and developing antibodies.

     Two of our antibody product candidates are undergoing clinical trials that
test for safety, dosing and effectiveness in humans.  These antibodies target
specific diseases that arise when the human immune system induces undesired
inflammation.  These antibodies are designed to block a component of the human
immune system that causes undesired inflammation while allowing beneficial
components of the immune system to remain functional.  The specific component of
the human immune system which these two product candidates are designed to block
is called "complement."

     We call one of the antibodies that is in clinical trials pexelizumab or
5G1.1-SC.  Pexelizumab completed the testing of safety and efficacy in a Phase
IIb clinical trial for the treatment of acute inflammation in patients caused by
the trauma of heart and lung bypass procedures, cardiopulmonary bypass surgery
or CPB.  Pending a full evaluation of the data, and in conjunction with planned
discussions with the FDA or Food and Drug Administration, we expect to initiate
a Phase III efficacy and safety trial with pexelizumab in coronary artery bypass
graft surgery or CABG patients at the earliest possible opportunity.  Initiation
of this Phase III trial, which would be conducted with our collaborator Proctor
& Gamble Pharmaceuticals, is subject to many factors that we do not control and
cannot predict.  In September 2000, we also announced that we had received "fast
track" designation from the FDA for pexelizumab in the treatment of patients
undergoing CPB.  Product candidates with "fast track" designation are eligible
for expedited development and FDA review.

     Pexelizumab is also in two additional Phase II clinical trials testing the
safety and efficacy of pexelizumab for the treatment of acute inflammation in
patients caused by heart attacks or myocardial infarctions.  One study is in

                                       1


patients receiving thrombolytic therapy, and the other is in patients receiving
angioplasty, procedures for unblocking clogged arteries.

     We call the second antibody product candidate that is in clinical trials
5G1.1.  5G1.1 is in clinical development for the treatment of a variety of
chronic autoimmune diseases.  We completed a Phase II clinical study testing the
safety and efficacy of 5G1.1 in patients with rheumatoid arthritis, a chronic
autoimmune disease.  Pending a full evaluation of the interim data and final
six-month safety data from this trial, and in conjunction with planned
discussions with the FDA, we expect to initiate a Phase III efficacy and safety
trial with 5G1.1 in rheumatoid arthritis at the earliest possible opportunity.
Initiation of this Phase III trial is subject to many factors that we do not
control and cannot predict.  5G1.1 is also being tested in a Phase II trial for
the treatment of membranous nephritis, a kidney disease. In both rheumatoid
arthritis and membranous nephritis, open label twelve-month extension studies to
test long-term safety are on-going.  We are also testing 5G.1.1 in three
separate on-going Phase Ib clinical trials in patients for the treatment of
psoriasis, a skin disorder, dermatomyositis, a severe inflammatory muscle
disorder, and pemphigoid, a severe inflammatory skin disorder.  In October 2000,
we announced that the FDA granted Orphan Drug status to 5G1.1 for the treatment
of patients with dermatomyositis.  The Orphan Drug designation would provide us
with market exclusivity for this indication for seven years from the drug's
approval date.

     We have retained all of our rights to 5G1.1.  We have a collaboration
agreement with Procter & Gamble Pharmaceuticals with respect to the development
and commercialization of pexelizumab. The initial subject of the collaboration
is to study the use of pexelizumab for the treatment of inflammation caused by
heart and lung bypass procedures and inflammation during heart attacks in
association with procedures for unblocking clogged heart arteries.

     In addition to our program for developing products that inhibit the
inflammatory effects of complement and our technology programs focusing on human
antibody discovery and development, we are developing another type of anti-
inflammatory drug known as Apogens. Apogens are designed to block disease-
causing T-cells, another component of the human immune system. We are currently
completing preclinical studies of our first Apogen, targeting the treatment of
patients with multiple sclerosis.

     We are also developing methods of blocking the human immune system to
permit the use of cells and organs from non-human species in the treatment of
diseases in humans. This product development program is initially targeting the
treatment of patients with Parkinson's disease and patients with spinal cord
injury with genetically altered pig cells.

     In September 2000, we acquired Prolifaron, Inc., a privately held
biopharmaceutical company located in San Diego, California.  Prolifaron was
developing therapeutic antibodies addressing multiple diseases, including
cancer.  The acquisition was in the form of a merger of our wholly owned
subsidiary, Alexion Antibody Technologies, Inc. and Prolifaron.  In the merger,
we issued 355,594 shares of our common stock, to the stockholders of Prolifaron
and we are obligated to issue up to 44,364 shares of our common stock upon the
exercise of fully vested options granted under Prolifaron's stock option plan.
We accounted for the acquisition of Prolifaron using the purchase method of
accounting through the issuance of common stock and fully vested options having
an aggregate fair value of approximately $43.9 million.  We agreed to register
the possible resale of those shares of common stock by the Prolifaron
stockholders.  We filed a registration statement with the SEC covering the
resale of approximately half of the shares in December 2000.  That registration
statement became effective on January 5, 2001 (Registration No. 333-52886).
Under the terms of the merger agreement, we are required to file a registration
statement with the SEC covering the resale of the remaining shares on or before
April 30, 2001, for which this registration statement is intended.

     Through Alexion Antibody Technologies, we have developed the important
additional capabilities to discover and develop additional antibody product
candidates for the treatment of inflammatory diseases and cancer.  Antibodies in
preclinical development include a platelet targeting antibody, an antagonist of
a specific growth factor for brain cancers, an antibody that specifically
targets a subset of blood tumor cells, as well as a catalytic antibody for
broad-based prodrug chemotherapy of many types of cancers.  The catalytic
antibody was exclusively licensed from The Scripps Research Institute.

     We were incorporated in Delaware in January 1992.  Our principal executive
offices are located at 352 Knotter Drive, Cheshire, Connecticut 06410, and our
telephone number is (203) 272-2596.

                                       2


                                  RISK FACTORS

     You should carefully consider the following risk factors before you decide
to invest in the securities. You should also consider the other information in
this prospectus and information incorporated by reference in this prospectus.
The risks and uncertainties below are not the only ones facing Alexion.  We are
also subject to additional risks and uncertainties not presently known to us. If
any of these risks actually occurs, our business, financial condition, operating
results or cash flows could be harmed.

If we continue to incur operating losses, we may be unable to continue our
operations.

     We have incurred losses since we started our company in January 1992. As of
January 31, 2001, we had an accumulated deficit of approximately $101.4 million.
If we continue to incur operating losses and fail to become a profitable
company, we may be unable to continue our operations. Since we began our
business, we have focused on research and development of product candidates. We
have no products that are available for sale. We expect to continue to operate
at a net loss for at least the next several years as we continue our research
and development efforts, continue to conduct clinical trials and develop
manufacturing, sales, marketing and distribution capabilities. Our future
profitability depends on our receiving regulatory approval of our product
candidates and our ability to successfully manufacture and market approved
drugs. The extent of our future losses and the timing of our profitability, if
we are ever profitable, are highly uncertain.

If we do not obtain regulatory approval for our drug products we will not be
able to sell our drug products.

     We cannot sell or market our drugs without regulatory approval. If we do
not obtain regulatory approval for our products, the value of our company and
our results of operations will be harmed. In the United States, we must obtain
approval from the U.S. Food and Drug Administration, or FDA, for each drug that
we intend to sell. Obtaining FDA approval is typically a lengthy and expensive
process, and approval is highly uncertain. Foreign governments also regulate
drugs distributed outside the United States, whose approval can also be lengthy,
expensive and highly uncertain. None of our product candidates has received
regulatory approval to be marketed and sold in the United States or any other
country. We do not anticipate receiving regulatory approval of any of our
product candidates for at least the next several years.

If our drug trials are delayed or achieve unfavorable results, we will have to
delay or may be unable to obtain regulatory approval for our products.

     We must conduct extensive testing of our product candidates before we can
obtain regulatory approval for our products. We need to conduct both preclinical
animal testing and clinical human trials. These tests and trials may not achieve
favorable results. We would need to reevaluate any drug that did not test
favorably and either alter the study, the drug or the dose, or abandon the drug
development project. In such circumstances, we would not be able to obtain
regulatory approval on a timely basis, if ever.

     We have announced the completion of a Phase IIb trial of pexelizumab and
completion of a Phase II trial of 5G1.1.  Completion of these trials does not
guarantee that we will initiate additional trials for our product candidates,
that if initiated the trials will be completed, or that if completed the trials
will be sufficient to apply for or receive regulatory approvals.

     There are many reasons why drug testing could be delayed or terminated. For
human trials, patients must be recruited and each product candidate must be
tested for each clinical indication, at various doses and formulations. Also, to
ensure safety and effectiveness, the effect of drugs often must be studied over
a long period of time, especially for the chronic diseases that we are studying.
Unfavorable results or insufficient patient enrollment in our clinical trials
could delay or cause us to abandon a product development program.

     Additional factors that can cause delay or termination of our clinical
trials include:

     .  slow patient enrollment;

     .  long treatment time required to demonstrate effectiveness;

     .  lack of sufficient supplies of the product candidate;

     .  adverse medical events or side effects in treated patients;

                                       3


     .  lack of effectiveness of the product candidate being tested; and

     .  lack of sufficient funds.

We may expand our business through new acquisitions that could disrupt our
business and harm our financial condition.

     Our business strategy includes expanding our products and capabilities, and
we may seek acquisitions to expand our business.  Acquisitions involve numerous
risks, including:

     .  substantial cash expenditures;
     .  potentially dilutive issuance of equity securities;
     .  incurrance of debt and contingent liabilities;
     .  difficulties in assimilating the operations of the acquired companies;
     .  diverting our management's attention away from other business concerns;
     .  risks of entering markets in which we have limited or no direct
        experience; and
     .  the potential loss of our key employees or key employees of the acquired
        companies.

     We cannot assure you that any acquisition will result in long-term benefits
to us or that our management will be able to manage the acquired businesses
effectively.  We may also incorrectly judge the value or worth of an acquired
company or business.  In addition, our future success would depend in part on
our ability to manage the rapid growth associated with some of these
acquisitions.  We cannot give assurances that we will be able to make the
combination of our business with that of acquired businesses or companies work
or be successful.  Furthermore, the development or expansion of our business or
any acquired business or companies may require a substantial capital investment
by us.  We may not have these necessary funds nor may they be readily available
to us on acceptable terms or at all.  We may also seek to raise funds by selling
shares of our stock, which could dilute your ownership interest in our company.

     On September 22, 2000, we purchased all of the capital stock and other
outstanding securities of Prolifaron, Inc., a privately held biopharmaceutical
company that is developing therapeutic antibodies addressing multiple disease,
including cancer, for approximately 400,000 shares of our outstanding capital
stock.  We cannot give assurances that the integration of our businesses with
the businesses of Prolifaron will be successful or that we will be able to
achieve the synergies we anticipate.

If we fail to obtain the capital necessary to fund our operations, we will be
unable to continue or complete our product development.

     In the future, we will need to raise substantial additional capital to fund
operations and complete our product development. We may not get funding when we
need it or on favorable terms. If we cannot raise adequate funds to satisfy our
capital requirements, we may have to delay, scale-back or eliminate our research
and development activities or future operations. We might have to license our
technology to others. This could result in sharing revenues which we might
otherwise retain for ourselves. Any of these actions may harm our business.  The
amount of capital we may need depends on many factors, including:

     .  the progress, timing and scope of our research and development programs;

     .  the progress, timing and scope of our preclinical studies and clinical
        trials;

     .  the time and cost necessary to obtain regulatory approvals;

     .  the time and cost necessary to further develop manufacturing processes,
        arrange for contract manufacturing or build manufacturing facilities and
        obtain the necessary regulatory approvals for those facilities;

     .  the time and cost necessary to develop sales, marketing and distribution
        capabilities; and

     .  any new collaborative, licensing and other commercial relationships that
        we may establish.

If our collaboration with Procter & Gamble is terminated, we may be unable to
commercialize pexelizumab or 5G1.1-SC in the time expected, if at all, and our
business

                                       4


would be harmed.

     We rely exclusively on Procter & Gamble to provide funding and additional
resources for the development and commercialization of pexelizumab or 5G1.1-SC.
These include funds and resources for:

     .  clinical development and clinical and commercial manufacturing;

     .  obtaining regulatory approvals; and

     .  sales, marketing and distribution efforts worldwide.

     We cannot guarantee that Procter & Gamble will devote the resources
necessary to successfully develop and commercialize pexelizumab. Either party
may terminate our collaboration agreement for specified reasons, including a
material breach.

     Termination of our agreement with Procter & Gamble would cause significant
delays in the development of pexelizumab and result in additional development
costs. We would need to fund the development and commercialization of
pexelizumab on our own or identify a new development partner. We might also have
to repeat testing already completed with Procter & Gamble.

If we are unable to engage and retain third-party collaborators, our research
and development efforts may be delayed.

     We depend upon third-party collaborators to assist us in the development of
our product candidates. If any of our existing collaborators breaches or
terminates its agreement with us or does not perform its development work under
the agreement, we would experience significant delays in the development or
commercialization of our product candidates. We would also experience
significant delays if we cannot engage additional collaborators when required.
In either event, we would be required to devote additional funds or other
resources to these activities or to terminate them.

     We cannot assure you that:

     .  we will be able to negotiate acceptable collaborative agreements to
        develop or commercialize our products;

     .  any arrangements with third parties will be successful; or

     .  current or potential collaborators will not pursue treatments for other
        diseases or seek other ways of developing treatments for our disease
        targets.

If we cannot protect the confidentiality and proprietary nature of our trade
secrets, our business and competitive position will be harmed.

     Our business requires using sensitive technology, techniques and
proprietary compounds which we protect as trade secrets. However, since we are a
small company, we also rely heavily on collaboration with suppliers, outside
scientists and other drug companies. Collaboration presents a strong risk of
exposing our trade secrets. If our trade secrets were exposed, it would help our
competitors and adversely affect our business prospects.

     In order to more effectively protect our drugs and technology, we need to
obtain patents covering the drugs and technologies we develop. Our drugs are
expensive and time-consuming to test and develop. Without patent protection,
competitors may copy our methods, or the chemical structure or other aspects of
our drug. Even if we obtain patents, the patents may not be broad enough to
protect our drugs from copy-cat products.

If we are found to be infringing on patents owned by others, we may be forced to
obtain a license to continue the sale or development of our drugs and/or pay
damages.

     Parts of our technology, techniques and proprietary compounds and potential
drug candidates may conflict with patents owned by or granted to others. If we
cannot resolve these conflicts, we may be liable for damages, be required to
obtain costly licenses or be stopped from manufacturing, using or selling our
products or conducting other activities. For example, we are aware of broad
patents owned by others relating to the manufacture, use and sale of recombinant
humanized antibodies, recombinant humanized single

                                       5


chain antibodies, recombinant human antibodies, recombinant human single chain
antibodies, and genetically engineered animals. Many of our products are
genetically engineered antibodies, including recombinant humanized antibodies,
recombinant humanized single chain antibodies, recombinant human antibodies, and
recombinant human single chain antibodies, and other products are tissues from
genetically engineered animals.

     We have received notices from the owners of some of these patents claiming
that their patents may be relevant to the development of some of our drug
candidates. In response to some of these notices, we have obtained licenses.
However, with regard to other patents, we have either determined in our judgment
that:

     .  our products do not infringe the patents;

     .  we do not believe the patents are valid; or

     .  we have identified and are testing various modifications which we
        believe should not infringe the patents and which should permit
        commercialization of our product candidates.

     Any patent holders could sue us for damages and seek to prevent us from
manufacturing, selling or developing our drugs. Legal disputes can be costly and
time consuming to defend. If any of these actions are successful, we could be
required to pay damages or to obtain a license to sell or develop our drugs. A
required license may not be available on acceptable terms, if at all.

If the testing or use of our products harms people, we could be subject to
costly and damaging product liability claims.

     The testing, manufacturing, marketing and sale of drugs for use in humans
exposes us to product liability risks. Side effects and other problems from
using our products could give rise to product liability claims against us. We
might have to recall our products, if any, from the marketplace. Some of these
risks are unknown at this time. For example, little is known about the potential
long-term health risks of transplanting pig tissue into humans, a goal of our
UniGraft product development program. Use of C5 Complement Inhibitors, such as
5G1.1 and pexelizumab, is associated with an increased risk for infection with
Neisseria bacteria.  Serious cases of Neisseria infection can result in brain
damage or death.

     In addition, we may be sued by people who participate in our clinical
trials. A number of patients who participate in such trials are already
critically ill when they enter a study. Any informed consents or waivers
obtained from people who sign up for our trials may not protect us from
liability or litigation. Our product liability insurance may not cover all
potential liabilities or may not completely cover any covered liabilities.
Moreover, we may not be able to maintain our insurance on acceptable terms. As a
result of these factors, a product liability claim, even if successfully
defended, could have a material adverse effect on our business, financial
condition or results of operations.

If we cannot manufacture our drug candidates in sufficient amounts at acceptable
costs and on a timely basis, we may be unable to have the necessary materials
for product testing, and later for potential sale in the market. Either event
would harm our business.

     For our drug trials, we need to produce sufficient amounts of product for
testing. Our small manufacturing plant cannot manufacture enough of our product
candidates for later stage clinical development. In addition, we do not have the
capacity to produce more than one product candidate at a time. We depend on a
few outside suppliers for manufacturing. If we experience interruptions in the
manufacture of our products for testing, our drug development efforts will be
delayed. If any of our outside manufacturers stops manufacturing our products or
reduces the amount manufactured, we will need to find other alternatives. If we
are unable to find an acceptable outside manufacturer on reasonable terms, we
will have to divert our own resources to manufacturing which may not be
sufficient to produce the necessary quantity or quality of product. As a result,
our ability to conduct testing would be materially adversely affected.
Submission of products and new development programs for regulatory approval
would be delayed. Our competitive position and our prospects for achieving
profitability could be materially and adversely affected.

     Manufacture of drug products is highly regulated by the FDA and other
domestic and foreign authorities.  We cannot assure you that we or our third-
party collaborators will successfully comply with all of those regulations,
which would have a materially adverse effect on our business.

     We have no experience or capacity for manufacturing drug products in
volumes that would be necessary to support commercial sales. If we are unable to
establish and maintain commercial scale manufacturing within our planned time
and cost parameters, sales of our products and our financial performance would
be adversely affected.

                                       6


     We may encounter problems in any of the following areas as we attempt to
increase the scale, process or size of manufacturing:

     .  design, construction and qualification of manufacturing facilities that
        meet regulatory requirements;

     .  production yields from the manufacturing process;

     .  purity of our drug products;

     .  quality control and assurance;

     .  shortages of qualified personnel; and

     .  compliance with FDA and other domestic and foreign governmental
        regulations.

If our business and products, even after regulatory approval is obtained, fail
to comply with regulatory requirements, our ability to sell products and conduct
business will be harmed.

     Even if we receive regulatory approval for any product, our business will
always be subject to substantial regulation by the FDA and comparable foreign
regulatory agencies.  The discovery of previously unknown problems with a
product or its manufacture, or the determination that previously known problems
are more significant than previously believed, may result in restrictions on the
product or manufacturer, including withdrawal of the product from the market.
The consequences for failure to comply with applicable regulatory requirements
can be serious, resulting in:

     .  warning letters;

     .  fines and other civil penalties;

     .  suspended regulatory approvals;

     .  refusal to approve pending applications or supplements to approved
        applications;

     .  refusal to permit exports from the United States;

     .  product recalls;

     .  seizure of products;

     .  injunctions;

     .  operating restrictions;

     .   total or partial suspension of production; and/or

     .  criminal prosecutions.

     Any of these consequences could result in withdrawal of approval, or
require reformulation of the drug, additional preclinical testing or clinical
trials, changes in labeling of the product, and/or additional marketing
applications. We would be required to expend time and resources in correcting
the problem, including any adverse publicity associated with the problem, in
order to put the product back on the market, if possible. These delays and uses
of resources would hurt our business, profitability and reputation.

If we are unable to establish sales, marketing and distribution capabilities, or
to enter into agreements with third parties to do so, we will be unable to
successfully market and sell future drug products.

     We have no sales, marketing or distribution personnel or capabilities. If
we are unable to establish those capabilities, either by developing our own
capabilities or entering into agreements with others, we will not be able to
successfully sell our products. In that event, we will not be able to generate
significant revenues. We cannot guarantee that we will be able to hire the
qualified sales and marketing personnel we need. We may not be able to enter
into any marketing or distribution agreements on acceptable terms, if at all. We
are relying on Procter & Gamble for sales, marketing and distribution of
pexelizumab. Procter & Gamble, or any future collaborators, may not succeed at
selling, marketing or distributing any of our future drug products.

                                       7


If we are unable to obtain reimbursement from government health administration
authorities, private health insurers and other organizations for our future
products, our products may be too costly for regular use and our ability to
generate revenues would be harmed.

     Our future revenues and profitability will be affected by the continuing
efforts of governmental and private third-party payors to contain or reduce the
costs of health care through various means. If these entities refuse to provide
reimbursement with respect to our products or determine to provide a low level
of reimbursement, our products may be too costly for general use, or our
profitability may be adversely impacted. Any limitation on the use of our
products will have a material adverse effect on our ability to generate revenues
and achieve profitability. We expect a number of federal, state and foreign
proposals to control the cost of drugs through government regulation. We are
unsure of the form that any health care reform legislation may take or what
actions any of these authorities and private payors may take in response to the
proposed reforms. Therefore, we cannot precisely predict the effect of any
reform on our business.

Even if we successfully develop our products for transplanting animal cells into
humans, this technology may not be accepted by the market due to medical
concerns or unanticipated regulation.

     Our program for the development of animal cells for transplantation into
humans may never result in any therapeutic products. This technology is subject
to extensive clinical testing and we are not aware of any such technology that
has been approved for sale by the FDA or comparable foreign regulatory
authorities. Even if we succeed in developing these products, our products may
not be widely accepted by the medical community or third-party payors until more
facts are established and ethical consensus is reached regarding the use of
animal cells. In addition, concerns relating to the risk of introducing new
animal viruses to infect the human species through the transplantation process
may also create additional regulatory hurdles for FDA approval. If accepted, the
degree of acceptance may limit the size of the market for our products.
Moreover, due to the controversial nature of transplantation of animal cells
into humans generally, market prices for our securities, may be subject to
increased volatility.

If our competitors get to the marketplace before we do with better or cheaper
drugs, our drugs may not be profitable to sell or to continue to develop.

     Each of Avant Immunotherapeutics, Inc, Leukosite Inc., a subsidiary of
Millenium Pharmaceuticals, Inc., Tanox, Inc., Abbott Laboratories, Baxter
Healthcare Corporation, Gliatech Inc. and Biocryst Pharmaceuticals have publicly
announced their intentions to develop drugs which target the inflammatory
effects of complement in the immune system. We are also aware that Pfizer, Inc.,
SmithKline Beecham Plc and Merck & Co., Inc. are also attempting to develop
complement inhibitor therapies. Each of Cambridge Antibody Technology, PLC,
MorphoSys AG and Dyax Corporation have publicly announced intentions to develop
therapeutic human antibodies from libraries of human antibody genes.
Additionally, each of Abgenix Inc. and Medarex, Inc. have publicly announced
intentions to develop therapeutic human antibodies from mice that have been bred
to include some human antibody genes. These and other large pharmaceutical
companies with significantly greater resources than ours, may develop,
manufacture and market better or cheaper drugs than our product candidates. They
may establish themselves in the marketplace before we are able to even finish
our clinical trials. Larger pharmaceutical companies also compete with us to
attract academic research institutions as drug development partners, including
for licensing these institutions' proprietary technology. If our competitors
successfully enter into such arrangements with academic institutions, we will be
precluded from pursuing those specific unique opportunities and may not be able
to find equivalent opportunities elsewhere.

If we fail to recruit and retain personnel, our research and product development
programs may be delayed.

     We are highly dependent upon the efforts of our senior management and
scientific personnel. There is intense competition for qualified scientific and
technical personnel. Since our business is very science-oriented and
specialized, we need to continue to attract and retain such people. We may not
be able to continue to attract and retain the qualified personnel necessary for
developing our business. If we lose the services of, or fail to recruit, key
scientific and technical personnel, our research and product development
programs would be significantly and detrimentally affected.

                                       8


     In particular, we highly value the services of Dr. Leonard Bell, our
President and Chief Executive Officer. The loss of his services could materially
and adversely affect our ability to achieve our development objectives.

The rights that have been and may in the future be granted to our stockholders
may frustrate attempts by others to take over our company.

     We have in place a shareholder rights plan, or "poison pill," which enables
our board of directors to issue rights to purchase preferred stock when someone
acquires 20% or more of the outstanding shares of our common stock. As a result
of the plan, anyone wishing to take over the company would most likely be forced
to negotiate a transaction with the company in order not to trigger the pill. If
we refused to negotiate, or negotiations were unsuccessful, a proposed takeover
could be frustrated. This would prevent our stockholders from participating in a
takeover or tender offer which might be of substantial value to them.

     In addition, under our certificate of incorporation, our board of directors
is authorized to issue one or more series of preferred stock with rights and
preferences determined by the board. The preferences and rights of any preferred
stock may be superior to those of the holders of our common stock. By issuing
preferred stock with superior rights to the common stock, the board could
frustrate a person who wishes to take over the company through a tender offer
for the outstanding common stock. These provisions are also intended to
encourage any person interested in acquiring us to negotiate with and obtain the
approval of our board of directors. These provisions could also delay, deter or
frustrate a merger or change in control.


               SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

     This prospectus contains some "forward-looking statements" as defined in
the Private Securities Litigation Reform Act of 1995 and information relating to
us that are based on the beliefs of our management, as well as assumptions made
by and the information currently available to our management. When used in this
prospectus, the words "estimate," "project," "believe," "anticipate," "intend,"
"expect" and similar expressions are intended to identify forward-looking
statements. These statements reflect our current views with respect to future
events and are subject to risks and uncertainties that could cause actual
results to differ materially from those contemplated in these forward-looking
statements, including those risks discussed in this prospectus.

     You are cautioned not to place undue reliance on these forward-looking
statements, which speak only as of the date of this prospectus. Except for
special circumstances in which a duty to update arises when prior disclosure
becomes materially misleading in light of subsequent events, we do not intend to
update any of these forward-looking statements to reflect events or
circumstances after the date of this prospectus or to reflect the occurrence of
unanticipated events.

                                USE OF PROCEEDS

     We will not receive any proceeds from the sale of common stock by the
selling stockholders.

                                DIVIDEND POLICY

     We have never declared or paid any dividends on our stock.  We currently
anticipate that we will retain all future earnings to support our growth
strategy.  Accordingly, we do not anticipate paying cash dividends on our stock
in the foreseeable future.  The payment of any future dividends will be at the
discretion of our board of directors and will depend upon, among other things,
future earnings, operations, capital requirements, our general financial
condition, and general business conditions.


                              SELLING STOCKHOLDERS

     The following table sets forth information regarding the selling
stockholders' beneficial ownership of our common stock as of March 26, 2001.



                                               Number of Shares           Number of               Number of
                                               of Common Stock            Shares of               Shares of
Selling Stockholder (1)                       Beneficiary owned         Common Stock            Common Stock
-----------------------
                                                                                  


                                       9





                                              Prior to Offering       Registered Herein         Beneficially
                                              -----------------       -----------------          Owned After
                                                                                              this Offering (2)
                                                                                              -----------------
                                                                                  
Carlos F. Barbas                                     19,019                  9,509                   9,510
Shana M. Barbas                                      20,316                  9,509                  10,807
Carlos Barbas (TRSI)                                    201                    100                     101
John Bentley                                          5,187                  2,593                   2,594
Ernest Beutler and Bonnie Beutler
  TTEE Ernest Beutler and Bonnie M.
  Beutler Trust U/A DTE 12/4/69                       2,500                  1,764                     736
Katherine Bowdish (3)                                46,684                 41,496                   5,188
Dennis R. Burton                                     38,037                 19,018                  19,019
Christopher John Connolly and Rhonda
  Marie Connolly Family Trust                         3,187                  2,593                     594
Chris Cruce                                              70                     35                      35
Glen Dautherty                                       18,393                  9,196                   9,197
Beth Fawsett                                            103                     51                      52
James C. Gilstrap Trust dated 1/16/95                15,561                  7,780                   7,781
Ernie Huang                                           1,296                    648                     648
James E. Iverson and Patricia Iverson                30,689                 14,696                  15,993
Kasirer Yeladim Holdings, LLC                        10,374                  5,187                   5,187
August Kee                                               70                     35                      35
John R. Larson                                       13,832                  6,916                   6,916
Richard Alan Lerner, Nicole Green Lerner,
  Trustees U.D.T. dated 11/14/94                     50,141                 25,070                  25,071
Richard Lerner (TRSI)                                   201                    100                     101
Marshall Linn and Joyce Linn                          2,687                  2,593                      94
Lippman Living Trust DTD 8/11/89                     19,748                 10,374                   9,374
Benjamin List (TRSI)                                     57                     28                      29
Paul Mailander                                          282                    141                     141
Thomas McDermott                                         25                     25                       0
Doug Mertz                                              141                     70                      71
Katharine Tate Overton Trust,
  Katharine Overton Coady, TTEE                       7,874                  5,187                   2,687
Staats M. Pellett, Jr.                                7,187                  2,593                   4,594
Lavern Punke                                          5,187                  5,187                       0
Christoph Rader (TRSI)                                   21                     10                      11
Daniel B. Rifkin                                      1,764                  1,764                       0
Don Sandberg                                            211                    105                     106
Joy Sprink                                               70                     35                      35
Robin M. Thomas                                       5,187                  2,593                   2,594
Juergen Wagner (3)                                       72                     72                       0
Sue Wood                                                 70                     35                      35
Guofu Zhong (TRSI)                                       72                     36                      36
                                                    -------                -------                 -------
                                                    326,516                187,114                 139,372
                                                    =======                =======                 =======

-------------------------------------
(1)  The selling stockholders received their shares of common stock in
       connection with our acquisition of Prolifaron, Inc. on September 23,
       2000.

(2)  Assumes that all shares offered by each selling stockholder are sold in
       this offering.

                                       10


(3)  Had not previously registered shares in S-3 Registration statement filed on
       December 28, 2000.



                              PLAN OF DISTRIBUTION

     Pursuant to an agreement and plan of merger effective September 23, 2000,
we acquired Prolifaron, Inc., a privately held biopharmaceutical company located
in San Diego, California.  In the merger, we issued 355,594 shares of our common
stock to the former stockholders of Prolifaron and we are obligated to issue up
to 44,364 shares of our common stock upon the exercise of fully vested options
granted under Prolifaron's stock option plan.  We agreed to register the
possible resale of those shares of common stock by the Prolifaron stockholders.
We filed a registration statement with the SEC covering the resale of
approximately half of the shares in December 2000.  That registration statement
became effective on January 5, 2001 (Registration No. 333-52886).  We are
required to file a registration statement with the SEC covering the resale of
the remaining half of the shares on or before April 30, 2001, for which this
registration statement is intended.

     The shares of common stock are being registered to permit the resale of the
common stock by the holders thereof from time to time after the date of this
prospectus.  We have agreed, among other things, to bear all expenses, other
than underwriting discounts, selling commissions and fees and expenses of other
advisors to holders of the common stock, in connection with the registration and
sale of the common stock covered by this prospectus.

     We will not receive any of the proceeds from the offering of the shares of
common stock by the selling securityholders.  We have been advised by the
selling securityholders that the selling securityholders (and their donees and
pledgees) may sell all or a portion of the shares of common stock beneficially
owned by them and offered hereby from time to time on any exchange on which the
securities are listed on terms to be determined at the times of such sales.  The
selling securityholders may also make private sales directly or through a broker
or brokers.  Alternatively, any of the selling securityholders may from time to
time offer the shares of common stock beneficially owned by them through
underwriters, dealers or agents, who may receive compensation in the form of
underwriting discounts, commissions or concessions from the selling
securityholders and the purchasers of the shares of common stock for whom they
may act as agent.  The aggregate proceeds to the selling securityholders from
the shares of common stock offered by them hereby will be the purchase price of
such shares of common stock less discounts and commissions, if any.

     Our outstanding common stock is listed for trading on the Nasdaq National
Market.

     The shares of common stock may be sold from time to time in one or more
transactions at fixed offering prices, which may be changed, or at varying
prices determined at the time of sale or at negotiated prices.  Such prices will
be determined by the holders of such securities or by agreement between such
holders and underwriters or dealers who may receive fees or commissions in
connection therewith.

     In order to comply with the securities laws of certain states, if
applicable, the shares of common stock will be sold in such jurisdictions only
through registered or licensed brokers or dealers.  In addition, in certain
states the shares of common stock may not be sold unless they have been
registered or qualified for sale in the applicable state or an exemption from
the registration or qualification requirement is available and is complied with.

     The selling securityholders and any broker-dealers, agents or underwriters
that participate with the selling securityholders in the distribution of the
shares of common stock may be deemed to be "underwriters" within the meaning of
the Securities Act, in which event any commissions received by such broker-
dealers, agents or underwriters and any profit on the resale of the shares of
common stock purchased by them may be deemed to be underwriting commissions or
discounts under the Securities Act.

                                 LEGAL MATTERS

     Fulbright & Jaworski L.L.P. New York, New York will pass upon the validity
of the securities offered hereby and some other legal matters on behalf of
Alexion.

                                    EXPERTS

     The audited consolidated financial statements incorporated by reference in
this prospectus and elsewhere in the registration statement have been audited by
Arthur Andersen LLP, independent public accountants, as indicated

                                       11


in their report with respect thereto, and are included herein in reliance upon
the authority of said firm as experts in accounting and auditing in giving said
report.

                      WHERE YOU CAN FIND MORE INFORMATION

     We file annual, quarterly and special reports, proxy statements and other
information with the Securities and Exchange Commission.  You may read and copy
any reports, statements or other information filed by us at the Commission's
public reference room at Room 1024, Judiciary Plaza, 450 Fifth Street, N.W.,
Washington, D.C. 20549 and the regional offices of the Commission located at
Seven World Trade Center, 13th Floor, New York, New York  10048, and 500 West
Madison Street, Chicago, Illinois 60661.  Copies of such material can be also
obtained from the Public Reference Section of the Commission at 450 Fifth
Street, N.W., Washington, D.C. 20549, and its public reference rooms in New
York, New York and Chicago, Illinois, at prescribed rates.  Please call the
Commission at 1-800-SEC-0330 for further information on the public reference
rooms.  Copies of such information may also be inspected at the reading room of
the library of the National Association of Securities Dealers, Inc., 1735 K
Street, N.W., Washington, D.C. 20006.  Our filings with the Commission are also
available to the public from commercial document retrieval services and at the
Commission's web site at "http://www.sec.gov."

     The SEC allows us to "incorporate by reference" the information we file
with them, which means that we can disclose important information to you by
referring you to those documents.  The information incorporated by reference is
considered to be part of this prospectus, and information that we file later
with the SEC will automatically update and supersede this information.  We
incorporate by reference the documents listed below and any future filings we
will make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the
Securities Exchange Act of 1934 until the selling stockholders sell all their
shares of Alexion stock.

        (i)    our current reports on Form 8-K, filed on January 23, 2001 and
                  January 29, 2001;

        (ii)   our quarterly report on Form 10-Q for the quarter ended
                  January 31, 2001, filed on March 15, 2001;

        (iii)  our amended current report on Form 8-K/A, filed on November 20,
                  2000;

        (iv)   our current reports on Form 8-K, filed on September 25, 2000,
                  October 3, 2000 and October 27, 2000;

        (v)    our quarterly report on Form 10-Q for the quarter ended
                  October 31, 2000, filed on December 15, 2000;

        (vi)   our annual report on Form 10-K for the fiscal year ended July 31,
                  2000, filed on October 6, 2000;

        (vii)  our registration statement on Form 8-A, filed on February 21,
                  1997, as amended on October 6, 2000; and

        (viii) our registration statement on Form 8-A, filed on February 12,
                  1996.

     We will furnish without charge to you, upon written or oral request, a copy
of any or all of the documents described above, except for exhibits to such
documents, unless such exhibits are specifically incorporated by reference into
such documents.  Requests should be addressed to:  Alexion Pharmaceuticals,
Inc., 352 Knotter Drive, Cheshire, Connecticut 06410, (203) 272-2596, Attention:
David W. Keiser, Executive Vice President and Chief Operating Officer.

     You should rely only on the information incorporated by reference or
provided in this prospectus or any supplement.  We have not authorized anyone
else to provide you with different information.  The selling stockholders will
not make an offer of these shares in any state where the offer is not permitted.
You should not assume that information in this prospectus or any supplement is
accurate as of any date other than the date on the front of these documents.

                                       12


                                    PART II

Item 14.  Other Expenses of Issuance and Distribution.

     The following table sets forth Alexion Pharmaceuticals, Inc. (the
"Company") estimates (other than the SEC registration fee) of the expenses in
connection with the issuance and distribution of the securities being
registered.  None of the following expenses are being paid by the selling
stockholders.

     SEC registration fee................................$ 1,000
     Legal fees and expenses.............................$15,000
     Accounting fees and expenses........................$ 4,000
     Printing fees.......................................$ 4,000
     Miscellaneous expenses..............................$ 6,000
                                                         -------
   Total:                                                $30,000
                                                         =======

Item 15.  Indemnification of Directors and Officers.

     Section 145 of the Delaware General Corporation Law (the "DGCL") empowers a
Delaware corporation to indemnify any person who was or is a party or is
threatened to be made a party to any threatened, pending or completed action,
suit or proceeding, whether civil, criminal, administrative or investigative
(other than an action by or in the right of such corporation) by reason of the
fact that such person is or was a director, officer, employee or agent of such
corporation, or is or was serving at the request of such corporation as a
director, officer, employee or agent of another corporation or enterprise.  A
corporation may, in advance of the final disposition of any civil, criminal,
administrative or investigative action, suit or proceeding, pay the expenses
(including attorneys' fees) incurred by any officer, director, employee or agent
in defending such action, provided that the director or officer undertakes to
repay such amount if it shall ultimately be determined that he is not entitled
to be indemnified by the corporation.  A corporation may indemnify such person
against expenses (including attorneys' fees), judgments, fines and amounts paid
in settlement actually and reasonably incurred by such person in connection with
such action, suit or proceeding if he acted in good faith and in a manner he
reasonably believed to be in or not opposed to the best interests of the
corporation, and, with respect to any criminal action or proceeding, had no
reasonable cause to believe his conduct was unlawful.

     A Delaware corporation may indemnify officers and directors in an action by
or in the right of the corporation to procure a judgment in its favor under the
same conditions, except that no indemnification is permitted without judicial
approval if the officer or director is adjudged to be liable to the corporation.
Where an officer or director is successful on the merits or otherwise in the
defense of any action referred to above, the corporation must indemnify him
against the expenses (including attorneys fees) which he actually and reasonably
incurred in connection therewith.  The indemnification provided is not deemed to
be exclusive of any other rights to which an officer or director may be entitled
under any corporation's by-law, agreement, vote or otherwise.

     In accordance with Section 145 of the DGCL, Section EIGHTH of the Company's
Certificate of Incorporation, as amended (the "Certificate") provides that the
Company shall indemnify each person who is or was a director, officer, employee
or agent of the Company (including the heirs, executors, administrators or
estate of such person) or is or was serving at the request of the Company as a
director, officer, employee or agent of another corporation, partnership, joint
venture, trust or other enterprise, to the fullest extent permitted.  The
indemnification provided by the Certificate shall not be deemed exclusive of any
other rights to which any of those seeking indemnification or advancement of
expenses may be entitled under any by-law, agreement, vote of shareholders or
disinterested directors or otherwise, both as to action in his official capacity
and as to action in another capacity while holding such office, and shall
continue as to a person who has ceased to be a director, officer, employee or
agent and shall inure to the benefit of the heirs, executors and administrators
of such a person.  Expenses (including attorneys' fees) incurred in defending a
civil, criminal, administrative or investigative action, suit or proceeding
shall be paid by the Company in advance of the final disposition of such action,
suit or proceeding upon receipt of an undertaking by or on behalf of the
indemnified person to repay such amount if it shall ultimately be determined
that he is not entitled to be indemnified by the company.  Section NINTH of the
certificate provides that a director of the Company shall not be personally
liable to the Company or its stockholders for monetary damages for breach of
fiduciary duty as a director, except for liability (i) for any breach of the
director's duty of loyalty to the Company or its stockholders, (ii) for acts or
omissions not in good faith or which involve intentional misconduct or a knowing
violation of law, (iii) under Section 174 of the DGCL, or (iv) for any
transaction from which the director derived an improper personal benefit.

                                      II-1


     Insofar as indemnification for liabilities arising under the Securities Act
of 1933 may be permitted to directors, officers or persons controlling the
registrant pursuant to the foregoing provisions, the registrant has been
informed that in the opinion of the Securities and Exchange Commission, such
indemnification is against public policy as expressed in the Act and is
therefore unenforceable.

Item 16.  Exhibits and Financial Statement Schedules.


     (a)  Exhibits

 2.1 Agreement and Plan of Merger by and among Alexion Pharmaceuticals, Inc.,
PI Acquisition Company, Inc.and Prolifaron, Inc., dated as of September 22,
2000.*
 5.1  Opinion of Fulbright & Jaworski L.L.P. regarding legality. +
23.1  Consent of Fulbright & Jaworski L.L.P. (included in Exhibit 5.1). +
23.2  Consent of Arthur Andersen LLP. +
24.1  Power of Attorney.
__________________
*  Incorporated by reference on our report on Form 8-K, filed on October 3, 2000
+   Filed herewith

     (b)  Financial Statement Schedules.
               None.

Item 17.  Undertakings

     (a)  The undersigned Registrant hereby undertakes:

          (1)  To file, during any period in which offers or sales are being
     made, a post-effective amendment to this registration statement to include
     any material information with respect to the plan of distribution not
     previously disclosed in the registration statement or any material change
     to such information in the registration statement;

          (2)  That, for the purpose of determining any liability under the
     Securities Act of 1933, each such post-effective amendment shall be deemed
     to be a new registration statement relating to the securities offered
     therein, and the offering of such securities at that time shall be deemed
     to be the initial bona fide offering thereof;

          (3)  To remove from registration by means of a post-effective
     amendment any of the securities being registered which remain unsold at the
     termination of the offering.

     (b)  The undersigned Registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
Registrant's annual report pursuant to section 13(a) or section 15(d) of the
Securities Exchange Act of 1934 (and, where applicable, each filing of an
employee benefit plan's annual report pursuant to section 15(d) of the
Securities Exchange Act of 1934) that is incorporated by reference in the
registration statement shall be deemed to be a new registration statement
relating to the securities offered therein, and the offering of such securities
at that time shall be deemed to be the initial bona fide offering thereof.

     (c)  Insofar as indemnification for liabilities arising under the
Securities Act of 1933 may be permitted to directors, officers and controlling
persons of the Registrant pursuant to the foregoing provisions, or otherwise,
the Registrant has been advised that in the opinion of the Securities and
Exchange Commission such indemnification is against public policy as expressed
in the Act and is, therefore, unenforceable. In the event a claim for
indemnification against such liabilities (other than the payment by the
Registrant of expenses incurred or paid by a director, officer, or controlling
person of the registrant in the successful defense of any action, suit or
proceeding) is asserted by such director, officer, or controlling person of the
Registrant in connection with the securities being registered, the Registrant
will, unless in the opinion of its counsel the matter has been settled by
controlling precedent, submit to a court of appropriate jurisdiction the
question whether such indemnification by it is against public policy as
expressed in the Act and will be governed by the final adjudication of such
issue.

                                      II-2


                                   SIGNATURES

     Pursuant to the requirements of the Securities Act of 1933, as amended, the
registrant certifies that it has reasonable grounds to believe that it meets all
of the requirements for filing on Form S-3 and has duly caused this registration
statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Cheshire and State of Connecticut on the 27 day of
April, 2001.


                                        ALEXION PHARMACEUTICALS, INC.



                                        By:      /s/ LEONARD BELL
                                            ----------------------------------
                                                    Leonard Bell, M.D.
                                             President, Chief Executive Officer,
                                                    Secretary and Treasurer

                        _______________________________

                               POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that each individual whose signature
appears below constitutes and appoints Leonard Bell and David W. Keiser, or
either of them, his true and lawful attorney-in-fact and agent with full power
of substitution and resubstitution, for him and in his name, place and stead, in
any and all capacities, to sign any and all amendments (including post-effective
amendments) to this Registration Statement and to file the same with all
exhibits thereto and all documents in connection therewith, with the Securities
and Exchange Commission, granting said attorney-in-fact and agent and each of
them, full power and authority to do and perform each and every act and thing
requisite and necessary to be done in and about the premises, as fully to all
intents and purposes as he or she might or could do in person, hereby ratifying
and confirming all that said attorney-in-fact and agent or any of them, or their
or his substitute or substitutes, may lawfully do or cause to be done by virtue
hereof.

  Pursuant to the requirements of the Securities Act of 1933, as amended, this
Registration Statement has been signed by the following persons in the
capacities and on the dates indicated:





                   Name                                  Title                                    Date
                   ----                                  -----                                    ----
                                                                       
                                             President, Chief Executive Officer
By:            /s/ LEONARD BELL                  Secretary, Treasurer                   April 27, 2001
--------------------------------------------     and Director (principal
              Leonard Bell, M.D.                 executive officer)


                                             Executive Vice President and
              /s/ DAVID W. KEISER                Chief Operating Officer                April 27, 2001
--------------------------------------------     (principal financial officer)
                David W. Keiser

                                             Vice President of Finance and
              /s/ BARRY P. LUKE                  Administration (principal              April 27, 2001
-------------------------------------------      accounting officer)
                  Barry P. Luke

                                             Chairman of the Board of
              /s/ JOHN H. FRIED                  Directors                              April 27, 2001
-------------------------------------------
              John H. Fried, Ph.D.

              /s/ JERRY T. JACKSON           Director                                   April 27, 2001
-------------------------------------------
                 Jerry T. Jackson



                                      II-3





                   Name                                  Title                                    Date
                   ----                                  -----                                    ----
                                                                       

              /s/ MAX LINK                   Director                                   April 27, 2001
-------------------------------------------
              Max Link, Ph.D.

              /s/ JOSEPH A. MADRI            Director                                   April 27, 2001
-------------------------------------------
          Joseph A. Madri, Ph.D., M.D.

              /s/ R. DOUGLAS NORBY           Director                                   April 27, 2001
-------------------------------------------
                R. Douglas Norby

              /s/ ALVIN S. PARVEN            Director                                   April 27, 2001
-------------------------------------------
                 Alvin S. Parven


                                      II-4


                                 EXHIBIT INDEX





Exhibit
Number    Exhibit
-------   -------
       
 2.1      Agreement and Plan of Merger by and among Alexion Pharmaceuticals, Inc., PI Acquisition
          Company, Inc.and Prolifaron, Inc., dated as of September 22, 2000.*
 5.1      Opinion of Fulbright & Jaworski L.L.P. regarding legality.+
23.1      Consent of Fulbright & Jaworski L.L.P. (included in Exhibit 5.1).+
23.2      Consent of Arthur Andersen LLP.+
24.1      Power of Attorney.

_________________________
*  Incorporated by reference on our report on Form 8-K, filed on October 3, 2000
+  Filed herewith