EMPLOYMENT AGREEMENT

     This EMPLOYMENT AGREEMENT (the "Agreement") dated as of October 22, 1997,
by and between Alexion Pharmaceuticals, Inc., a Delaware corporation (the
"Company"), and David Keiser (the "Executive").


                               W I T N E S S E T H


     WHEREAS, the Company wishes to employ Executive in an executive capacity
and Executive is desirous of being so employed;

     NOW, THEREFORE, in consideration of the premises and the mutual covenants
and agreements herein contained, the parties hereto agree as follows:

     1. Employment, Duties and Acceptance.

     (a) The Company hereby employs the Executive for the Term (as hereinafter
defined), to render full-time services to the Company as Executive
Vice-President and Chief Operating Officer ("COO"), and to perform such duties
commensurate with such office as he shall reasonably be directed by the Board of
Directors (the "Board") of the Company to perform, which duties shall be
consistent with the provisions of the By-laws in effect on the date hereof that
relate to the duties of the COO and Executive Vice President. The Executive will
report directly to the President.

     (b) The Executive hereby accepts such employment and agrees to render the
services described above.

     2. Term of Employment.

     The term of the Executive's employment under this Agreement (the "Term")
commenced as of July 17, 1997 (the "Effective Date") and shall end on the third
anniversary thereof unless sooner terminated pursuant to Section 7 or 8 of this
Agreement. This Agreement shall not be renewed unless otherwise mutually agreed
by the parties.

     3. Compensation.

     (a) As full compensation for all services to be rendered pursuant to this
Agreement, the Company agrees to pay the Executive, during the Term, an annual
base salary of $178,000 for the first year of the Term and for each subsequent
year of the Term an amount to be determined by the Company, payable in such
installments as is the policy of the Company with respect to executive employees
of the Company (the "Salary").

     (b) Executive may receive bonuses on such dates, in such amounts and on
such other terms as may be determined by the Board of Directors in 


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its sole discretion. In the sole discretion of the Board of Directors, such
bonuses, if any, may be paid in the form of grants of stock of the Company or
non-qualified stock options, each granted pursuant to plans adopted by the
Company and approved by the Company's Board of Directors.

     (c) The Company shall pay or reimburse the Executive for all reasonable
expenses actually incurred or paid by him during the Term in the performance of
his services under this Agreement, upon presentation of expense statements or
vouchers or such other supporting information as it reasonably may require.

     (d) The Executive shall be eligible under any incentive plan, stock option
plan, stock award plan, bonus, participation or extra compensation plan,
relocation plan, pension, group insurance or other so-called "fringe" benefits
which the Company generally provides for its executives.

     4. Other Benefits.

     In addition to all other benefits contained herein, the Executive shall be
entitled to:

          (1) Payment of health, disability, and life insurance at regular
     rates; and

          (2) Vacation time of four weeks per year taken, subject to fulfillment
     of his duties hereunder, in accordance with the vacation policy of the
     Company during the Term.

     5. Confidentiality.

     The Executive agrees that the "Proprietary Information and Inventions
Agreement" annexed hereto as Exhibit A and made a part hereof shall be deemed to
be a part of this Employment Agreement.

     6. Non-Competition.

     (a) During the Term the Executive shall not (1) provide any services,
directly or indirectly, to any other business or commercial entity or (2)
participate in the formation of any business or commercial entity. For a period
of one year following the date of termination, if terminated by the Company for
Cause or by the Executive for any reason other than as provided in Section 8
hereof, the Executive shall not (1) provide any services, directly or
indirectly, to any other business or commercial entity engaged in the Company's
Field of Interest or (2) participate in the formation of any business or
commercial entity engaged in the Company's Field of Interest; provided, however,
that nothing contained in this Section 6 shall be deemed to prohibit the
Executive from acquiring, solely as an investment, shares of capital stock (or
other interests) of any corporation (or other entity) not exceeding 2% of such
corporation's (or other entity's) then outstanding shares of capital stock. The
"Company's Field of Interest" means the primary business of the Company as


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determined from time to time by the Board of Directors. This Section 6 shall be
subject to written waivers that may be obtained by the Executive from the
Company.

     (b) If the Executive commits a breach, or threatens to commit a breach, of
any of the provisions of this Section 6 or Exhibit A, the Company shall have the
right and remedy to have the provisions of this Agreement specifically enforced
by any court having equity jurisdiction, it being acknowledged and agreed that
any such breach or threatened breach will cause irreparable injury to the
Company and that money damages will not provide an adequate remedy to the
Company.

     (c) If any of the covenants contained in Section 5, 6 or 10, or any part
thereof, is hereafter construed to be invalid or unenforceable, the same shall
not affect the remainder of the covenant or covenants, which shall be given full
effect without regard to the invalid portions.

     (d) If any of the covenants contained in Section 5, 6 or 10, or any part
thereof, is held to be unenforceable because of the duration of such provision
or the area covered thereby, the parties agree that the court making such
determination shall have the power to reduce the duration and/or area of such
provision and, in its reduced form, such provision shall then be enforceable.

     (e) The parties hereto intend to and hereby confer jurisdiction to enforce
the covenants contained in Sections 5, 6 and 10 upon the courts of any state
within the geographical scope of such covenants. In the event that the courts of
any one or more of such states shall hold any such covenant wholly unenforceable
by reasons of the breadth of such scope or otherwise, it is the intention of the
parties hereto that such determination not bar or in any way affect the
Company's right to the relief provided above in the courts of any other states
within the geographical scope of such other covenants, as to breaches of such
covenants in such other respective jurisdictions, the above covenants as they
relate to each state being, for this purpose, severable into diverse and
independent covenants.

     7. Termination by the Company.

     (a) The Company may terminate this Agreement without Cause or if any one or
more of the following shall occur:

          (1) The Executive shall die during the Term; provided, however, that
     the Executive's legal representatives shall be entitled to receive his
     Salary through the last day of the month in which his death occurs.

          (2) The Executive shall become physically or mentally disabled so that
     he is unable substantially to perform his services hereunder for (a) a
     period of 120 consecutive days, or (b) for shorter periods aggregating 180
     days during any twelve-month period. Notwithstanding such disability the
     Company shall continue to pay the Executive his Salary through the date of
     such termination.

          (3) The Executive acts, or fails to act, in a manner that provides
     Cause for termination. For purposes of this Agreement, the term "Cause"


                                        3






     means (a) the Executive's indictment for, or conviction of, any crime or
     serious offense involving money or other property which constitutes a
     felony in the jurisdiction involved, (b) the Executive's wilful and
     continual neglect or failure to discharge his duties, responsibilities, and
     obligations as Executive Vice President and COO of the Company after notice
     and a reasonable opportunity to cure, (c) the Executive's wilful misconduct
     or wilful breach of his fiduciary duties to the Company in connection with
     the performance of his duties, (d) the Executive's violation of any of the
     non-competition provisions of Section 6 hereof or the Executive's breach of
     any confidentiality provisions contained in Exhibit A hereto or (e) any act
     of fraud or embezzlement by the Executive involving the Company or any of
     its Affiliates.

     (b) All determinations of Cause or termination pursuant this Section 7
shall be determined by the Board (excluding the Executive if he is at such time
a member of the Board).

     8. Termination by the Executive.

     The Executive may terminate this Agreement on written notice to the Company
in the event of a material breach of the terms of this Agreement by the Company
and such breach continues uncured for 30 days after notice of such breach is
first given; provided, however, it shall constitute the termination of this
Agreement if such breach is for the payment of money and continues uncured for
ten days after notice of such breach is given.

     9. Severance.

     If, within the Term of this Agreement, the Company terminates this
Agreement for any reason other than Cause, death, or disability, or if the
Executive terminates this Agreement pursuant to Section 8, then: (1) the Company
shall pay the Executive a lump sum cash payment (the "Severance Payment") equal
to the greater of (x) the annual salary for the remainder of the then current
year of employment and (y) six months salary at the annual rate for the then
current year of employment; and (2) for options granted to the Executive prior
to the date of termination, the Company shall accelerate the vesting schedule
for such options such that the number of such options vested on the day of
termination shall be equal to the number of such options vested if the Executive
were to have been continuously employed by the Company until the date twelve
months after the date of termination. After termination of employment for any
reason other than death of the Executive, the Company shall continue to provide
all benefits subject to COBRA at its expense for the maximum required COBRA
period.

     10. Indemnification.

     The Company shall indemnify the Executive, to the maximum extent permitted
by applicable law, against all costs, charges and expenses incurred or sustained
by him in connection with any action, suit or proceeding to which he may be made
a party by reason of his being an officer, director or employee of the Company
or of any subsidiary or affiliate of the Company. The Company shall provide, at
its expense, Directors and Officers insurance for the Employee in amounts


                                       4






reasonably satisfactory to the Executive to the extent available at reasonable
rates, which determination shall be made by the Board.

     11. Arbitration.

     Any controversy or claim arising out of or relating to this Agreement or
the breach thereof shall be settled by arbitration in the City of New York, in
accordance with the rules then existing of the American Arbitration Association
(three arbitrators), and judgment upon the award rendered may be entered in any
court having jurisdiction thereof. The parties shall be free to pursue any
remedy before the arbitration tribunal that they shall be otherwise permitted to
pursue in a court of competent jurisdiction. The award of the arbitrators shall
be final and binding. During the pendency of any arbitration or any dispute not
yet submitted to arbitration, the Company shall not be entitled to any offset
against payments, stock awards or other benefits due to the Executive under this
Agreement or otherwise.

     12. Notices.

     All notices, requests, consents and other communications required or
permitted to be given hereunder shall be in writing and shall be deemed to have
been duly given if sent by private overnight mail service (delivery confirmed by
such service), registered or certified mail (return receipt requested and
received), telecopy (confirmed receipt by return fax from the receiving party)
or delivered personally, as follows (or to such other address as either party
shall designate by notice in writing to the other in accordance herewith):


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           If to the Company:

           Alexion Pharmaceuticals, Inc.
           25 Science Park
           New Haven, Connecticut 06510

           Telephone: 203-776-1790
           Fax:       203-776-2089


           If to the Executive:

           David Keiser
           37 Georgetown Circle
           Madison, CT 06443

           Telephone: 203-421-5691

     13. General.

     (a) This Agreement shall be governed by and construed and enforced in
accordance with the laws of the State of Connecticut applicable to agreements
made and to be performed entirely in Connecticut.

     (b) This Agreement sets forth the entire agreement and understanding of the
parties relating to the subject matter hereof, and supersedes all prior
agreements, arrangements and understandings, written or oral, relating to the
subject matter hereof. No representation, promise or inducement has been made by
either party that is not embodied in this Agreement, and neither party shall be
bound by or liable for any alleged representation, promise or inducement not so
set forth.

     (c) This Agreement may be amended, modified, superseded, canceled, renewed
or extended, and the terms or covenants hereof may be waived, only by a written
instrument executed by the parties hereto, or in the case of a waiver, by the
party waiving compliance. The failure of a party at any time or times to require
performance of any provision hereof shall in no manner affect the right at a
later time to enforce the same. No waiver by a party of the breach of any term
or covenant contained in this Agreement, whether by conduct or otherwise, or any
one or more or continuing waivers of any such breach, shall constitute a waiver
of the breach of any other term or covenant contained in this Agreement.


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     (d) This Agreement shall be binding upon the legal representatives, heirs,
distributees, successors and assigns of the parties hereto.

     IN WITNESS WHEREOF, the parties have executed this Agreement as of the date
first above written.


ALEXION PHARMACEUTICALS, INC.


By /s/ LEONARD BELL
   ---------------------------------
   Leonard Bell


   /s/ DAVID KEISER
   -----------------------------
   David Keiser


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                                    EXHIBIT A


                          ALEXION PHARMACEUTICALS, INC.

               PROPRIETARY INFORMATION AND INVENTIONS AGREEMENT


     I recognize that ALEXION Pharmaceuticals, Inc., a Delaware corporation (the
"Company", which term includes any subsidiaries thereof), is engaged in multiple
aspects of pharmaceutical development and biotechnology.

     I understand that:

     A. As part of my employment by the Company I am expected to make new
contributions of value of the Company.

     B. My employment creates a relationship of confidence and trust between me
and the Company with respect to any information:

          (1) Applicable to the business of the Company and made known to me by
     the Company or learned by me during the period of my employment; or

          (2) Applicable to the business of any client, customer or strategic
     partner of the Company, which may be made known to me by the Company or by
     any client, customer or strategic partner of the Company, or learned by me
     during the period of my employment.

     C. The Company possesses and will continue to possess information that has
been created, discovered or developed, or has otherwise become known to the
Company (including without limitation information created, discovered, developed
or made known by or to me during the period of my employment by the Company),
and/or in which property or other rights have been assigned or otherwise
conveyed to the Company, which information has commercial value in the business
in which the Company is engaged and none of which is in the public domain except
through the breach by me or anyone else of a confidentiality duty. All of the
aforementioned information is hereinafter called "Proprietary Information." By
way of illustration, but not limitation, Proprietary Information includes
"Developments (as herein defined), data and know-how, techniques, marketing
plans and opportunities, cost and pricing data, strategies, forecasts and
customer lists. By way of illustration, but not limitation, "Developments"
includes developments, improvements, discoveries, trade secrets, technologies,
processes, research, methods, procedures, designs, models, testing systems,
research, assays, compounds, molecules, organisms, gene sequences, cell lines,
complement inhibitors and other re-agents (including the composition thereof),
uses of any of the foregoing, computer software and programs (including source
code and related documentation), test and/or experimental data and results,
specifications, laboratory notebooks, drawings and technical information and
materials.


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     D. As used herein, the period of my employment includes any time in which I
may be retained by the Company as a consultant.

     In consideration of my employment or continued employment, as the case may
be, and the compensation received by me from the Company from time to time, I
hereby agree as follows:

          1. Prior to entering the employ of the Company, I have terminated
     employment with one or more prior employers. I agree to indemnify and hold
     harmless the Company, its directors, officers and employees against any
     liabilities and expenses including reasonable attorneys' fees and amounts
     paid in settlement, incurred by any of them in connection with any claim by
     any of my prior employers that the termination of my employment with such
     employer, my employment by the Company, or use of any skills and knowledge
     by the Company is a violation of contract or law.

          2. All Proprietary Information shall be the sole property of the
     Company and its assigns, and the Company and its assigns shall be the sole
     owner of all patents, copyrights, trade secrets and trademarks and other
     rights in connection therewith. I hereby assign to the Company any and all
     rights I may have or acquire in all Proprietary Information. At all times,
     both during my employment by the Company and after its termination, I will
     keep in confidence and trust all Proprietary Information, and I will not
     use or disclose any Proprietary Information without the written consent of
     the Company, except as may be necessary in the ordinary course of
     performing my duties as an employee of the Company.

          3. During the period of my employment by the Company I will not,
     without the Company's express written consent, engage in any employment or
     activity in any competitive business, other than for the Company.

          4. I will promptly disclose to the Company, or any persons designated
     by it, all Developments, improvements, processes, techniques, know-how,
     data and Developments made or conceived or reduced to practice or learned
     by me, either alone or jointly with others, during the period of my
     employment by the Company which are related to or useful in the business of
     the Company, or result from use of premises owned, leased or contracted for
     by the Company (all said Developments, improvements, processes, techniques,
     know-how, data and documentation, shall be collectively hereinafter called
     "Know-how").

          5. I agree that all Know-how shall be the sole property of the Company
     and its assigns, and the Company and its assigns shall be the sole owner of
     all patents, copyrights, trademarks and other rights in connection
     therewith. I hereby assign to the Company any rights I may have or acquire
     in all Know-how.

          I understand that this paragraph 5 does not apply to Know-how for
     which no equipment, supplies, facility or trade secret information of the
     Company was used and which was developed entirely on my own time, and (a)
     which does not relate (1) to the business of the Company or (2) to the
     Company's actual or demonstrably 


                                       9






     anticipated research or development, or (b) which does not result from any
     work performed by me for the Company. I agree to execute any documents
     requested by the Company to effectuate the intent of this paragraph.

          6. The Company shall have the right (but shall not be obligated) to
     use, assert and/or apply for patent, copyright, trademark and other
     statutory or common law protection for any or all Know-how in any and all
     countries. I agree to assist the Company in every reasonable way without
     additional compensation (but at the Company's expense), to apply for,
     prosecute and obtain, and from time to time enforce, defend and protect,
     any and all patent, copyright, trademark and other statutory or common law
     protection for any of the Know-how in any and all countries. I shall,
     whether during or following my employment by the Company, at the Company's
     request and expense, but without additional compensation to me, execute any
     and all assignments, transfers, applications and other papers covering any
     Know-how which may be considered necessary or helpful by the Company in
     furtherance of the foregoing and/or to accomplish the assignment, transfer
     and/or license of any Know-how to persons designated by the Company.

          7. In the event of the termination of my employment by me or by the
     Company for any reason, I will deliver to the Company all documents,
     materials, compounds, samples, plasmids, proteins, probes and data of any
     nature pertaining to my work with the Company and I will not take with me
     any documents or data of any description or any reproduction of any
     description containing or pertaining to any Proprietary Information, and
     Know-how.

          8. I represent that to the best of my knowledge my performance of all
     the terms of this Agreement and as an employee of the Company does not and
     will not breach any agreement to keep in confidence proprietary information
     acquired by me in confidence or in trust prior to my employment by the
     Company. I have not entered into, and I agree I will not enter into, any
     agreement either written or oral in conflict herewith.

          9. I understand as part of the consideration for the offer of
     employment extended to me by the Company and of my employment or continued
     employment by the Company, that I have not brought and will not bring with
     me to the Company or use in the performance of my responsibilities at the
     Company any materials or documents of a former employer which are not
     generally available to the public, unless I have obtained written
     authorization from the former employer for their possession and use.

          Accordingly, this is to advise the Company that the only materials or
     documents of a former employer which are not generally available to the
     public that I have brought or will bring to the Company or have used or
     will use in my employment are identified on Schedule A attached hereto,
     and, as to each such item, I represent that I have obtained prior to the
     effective date of my employment with the Company written authorization for
     their possession and use in my employment with the Company.


                                       10






          I also understand that, in my employment with the Company, I am not to
     breach any obligation of confidentiality that I have to former employers,
     and I agree that I shall fulfill all such obligations during my employment
     with the Company.

          10. In the event that any provision herein would be held to be
     invalid, prohibited or unenforceable in any jurisdiction for any reason
     (including, but not limited to, any provision which may be held
     unenforceable because of the scope, duration or area of its applicability),
     unless narrowed by construction, this Agreement shall, as to such
     jurisdiction, be construed as if such invalid, prohibited or unenforceable
     provision had been more narrowly drawn so as not to be invalid, prohibited
     or unenforceable (and the court making any such determination as to any
     provision shall have the power to modify such scope, duration or area or
     all of them, and such provision shall then be applicable in such modified
     form in such jurisdiction only). If, notwithstanding the foregoing, any
     provision herein would be held to be invalid, prohibited or unenforceable
     in any jurisdiction for any reason, such provision, as to such
     jurisdiction, shall be ineffective to the extent of such invalidity,
     prohibition or unenforceability, without invalidating the remaining
     provisions of this Agreement or affecting the validity or enforceability of
     such provision in any other jurisdiction.

          11. By reason of the fact that irreparable harm would be sustained by
     the Company in the event that there is a breach by me of any of the terms,
     covenants and agreements set forth herein, in addition to any other rights
     that the Company may otherwise have, the Company shall be entitled to apply
     to any court of competent jurisdiction and obtain specific performance
     and/or injunctive relief against me, without making a showing that monetary
     damages would be inadequate and without the requirement of posting any bond
     or other security whatsoever, in order to enforce or prevent any breach or
     threatened breach of any of the terms, covenants and agreements set forth
     herein, and I will not object thereto.

          12. This Agreement shall be binding upon me, my heirs, executors,
     assigns and administrators and shall inure to the benefit of the Company,
     its successors and assigns.

     This Agreement shall be governed in all respects by the laws of the State
of Delaware, applicable to agreements made and to be performed solely therein.


                                          By: /s/ 
                                              ----------------------------------
                                              

Dated: Oct. 22, 1997



ACCEPTED AND AGREED TO:
ALEXION PHARMACEUTICALS, INC.


By: /s/
    -------------------------------
    
    Title: President/CEO


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