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                                                                    Exhibit 10.2


                          REGISTRATION RIGHTS AGREEMENT

              This Registration Rights Agreement (the "Agreement") is dated as
of March 8, 2000, by and among ALEXION PHARMACEUTICALS, INC., a Delaware
corporation (the "Company"), J.P. MORGAN SECURITIES INC., U.S. BANCORP PIPER
JAFFRAY INC., CHASE SECURITIES INC. and WARBURG DILLON READ LLC (individually,
an "Initial Purchaser," and together, the "Initial Purchasers").

              This Agreement is entered into in connection with the Purchase
Agreement, dated as of March 3, 2000 (the "Purchase Agreement"), by and among
the Company and the Initial Purchasers, which provides for the sale by the
Company to the Initial Purchasers of $120,000,000 aggregate principal amount of
the Company's 5 3/4% Convertible Subordinated Notes Due 2007 (the "Firm Notes"),
which are convertible into Common Stock of the Company, par value $.0001 per
share (the "Underlying Shares"), plus up to an additional $30,000,000 aggregate
principal amount of the same which the Initial Purchasers may subsequently elect
to purchase pursuant to the terms of the Purchase Agreement (the "Additional
Notes" and together with the Firm Notes, the "Convertible Notes"). The Notes are
being issued pursuant to an indenture dated as of the date hereof (the
"Indenture"), between the Company and The Chase Manhattan Bank, as Trustee.

              In order to induce the Initial Purchasers to enter into the
Purchase Agreement, the Company has agreed to provide the registration rights
set forth in this Agreement for the benefit of the Initial Purchasers and any
subsequent holder or holders of the Convertible Notes or Underlying Shares. The
execution and delivery of this Agreement is a condition to the Initial
Purchasers' obligation to purchase the Firm Notes under the Purchase Agreement.

              The parties hereby agree as follows:

1.       Definitions.

              As used in this Agreement, the following terms shall have the
following meanings:

              AGREEMENT: See the first introductory paragraph hereto.

              AMOUNT OF REGISTRABLE SECURITIES: (a) With respect to Convertible
Notes constituting Registrable Securities, the aggregate principal amount of all
such Convertible Notes outstanding, (b) with respect to Underlying Shares
constituting Registrable Securities, the aggregate number of such Underlying
Shares outstanding multiplied by the Conversion Price (as defined in the
Indenture relating to the Convertible Notes upon the conversion of which such
Underlying Shares were issued) in effect at the time of computing the Amount of
Registrable Securities or, if no such Convertible Notes are then outstanding,
the last Conversion Price that was in effect under such Indenture when any such
Convertible Notes were last outstanding,


<PAGE>



and (c) with respect to combinations thereof, the sum of (a) and (b) for the
relevant Registrable Securities.

              BUSINESS DAY: Any day that is not a Saturday, Sunday or a day on
which banking institutions in New York are authorized or required by law to be
closed.

              CLOSING DATE: March 8, 2000.

              COMPANY: See the first introductory paragraph hereto.

              CONVERTIBLE NOTES: See the second introductory paragraph hereto.

              DAMAGES PAYMENT DATE: See Section 3(c) hereof.

              DEPOSITARY: The Depository Trust Company until a successor is
appointed by the Company.

              EFFECTIVENESS DATE: The 180th day after the Closing Date.

              EFFECTIVENESS PERIOD: See Section 2 hereof.

              EXCHANGE ACT: The Securities Exchange Act of 1934, as amended, and
the rules and regulations of the SEC promulgated thereunder.

              FILING DATE: The 90th day after the Closing Date.

              HOLDER: Any holder of Registrable Securities.

              INDEMNIFIED HOLDER: See Section 6 hereof.

              INDEMNIFIED PERSON: See Section 6 hereof.

              INDEMNIFYING PERSON: See Section 6 hereof.

              INDENTURE: See the second introductory paragraph hereto.

              INITIAL PURCHASERS: See the first introductory paragraph hereto.

              INITIAL SHELF REGISTRATION: See Section 2(a) hereof.

              INSPECTORS: See Section 4(n) hereof.

              LIQUIDATED DAMAGES: See Section 3(a) hereof.

              NASD: See Section 4(q) hereof.





                                       -2-
<PAGE>

              PERSON: An individual, partnership, corporation, limited liability
company, unincorporated association, trust or joint venture, or a governmental
agency or political subdivision thereof.

              PROSPECTUS: The prospectus included in any Registration Statement
(including, without limitation, any prospectus subject to completion and a
prospectus that includes any information previously omitted from a prospectus
filed as part of an effective registration statement in reliance upon Rule 430A
promulgated under the Securities Act), as amended or supplemented by any
prospectus supplement, and all other amendments and supplements to the
Prospectus, including post-effective amendments, and all material incorporated
by reference or deemed to be incorporated by reference in such Prospectus.

              PURCHASE AGREEMENT: See the second introductory paragraph hereto.

              RECORDS: See Section 4(n) hereof.

              REGISTRABLE SECURITIES: All Convertible Notes and all Underlying
Shares upon original issuance thereof and at all times subsequent thereto until
the earliest to occur of (i) a Registration Statement covering such Convertible
Notes and Underlying Shares having been declared effective by the SEC and such
Convertible Notes and Underlying Shares have been disposed of in accordance with
such effective Registration Statement, (ii) such Convertible Notes and
Underlying Shares having been sold in compliance with Rule 144 or could (except
with respect to affiliates of the Company within the meaning of the Securities
Act) be sold in compliance with Rule 144(k), or (iii) such Convertible Notes and
any Underlying Shares cease to be outstanding.

              REGISTRATION DEFAULT: See Section 3(a) hereof.

              REGISTRATION STATEMENT: Any registration statement of the Company
filed with the SEC pursuant to the provisions of this Agreement, including the
Prospectus, amendments and supplements to such registration statement, including
post-effective amendments, all exhibits and all material incorporated by
reference or deemed to be incorporated by reference in such registration
statement.

              RULE 144: Rule 144 promulgated under the Securities Act, as such
Rule may be amended from time to time, or any similar rule (other than Rule
144A) or regulation hereafter adopted by the SEC providing for offers and sales
of securities made in compliance therewith resulting in offers and sales by
subsequent holders that are not affiliates of an issuer of such securities being
free of the registration and prospectus delivery requirements of the Securities
Act.

              RULE 144A: Rule 144A promulgated under the Securities Act, as such
Rule may be amended from time to time, or any similar rule (other than Rule 144)
or regulation hereafter adopted by the SEC.


                                       -3-
<PAGE>

              RULE 415: Rule 415 promulgated under the Securities Act, as such
Rule may be amended from time to time, or any similar rule or regulation
hereafter adopted by the SEC.

              SEC: The Securities and Exchange Commission.

              SECURITIES ACT: The Securities Act of 1933, as amended, and the
rules and regulations of the SEC promulgated thereunder.

              SHELF REGISTRATION: See Section 2(b) hereof.

              SHELF REGISTRATION STATEMENT: See Section 2(b) hereof.

              SUBSEQUENT SHELF REGISTRATION: See Section 2(b) hereof.

              TIA: The Trust Indenture Act of 1939, as amended, and the rules
and regulations of the SEC promulgated thereunder.

              TRUSTEE: The Trustee under the Indenture.

              UNDERLYING SHARES: See the second introductory paragraph hereto.

              UNDERWRITTEN REGISTRATION OR UNDERWRITTEN OFFERING: A registration
in which securities of the Company are sold to an underwriter for reoffering to
the public.

2.       SHELF REGISTRATION

              (a) SHELF REGISTRATION. The Company shall as promptly as
reasonably practicable file with the SEC a Registration Statement for an
offering to be made on a continuous basis pursuant to Rule 415 covering all of
the Registrable Securities (the "INITIAL SHELF REGISTRATION"). The Company shall
use its reasonable best efforts to file with the SEC the Initial Shelf
Registration on or prior to the Filing Date. The Initial Shelf Registration
shall be on Form S-3 or another appropriate form permitting registration of such
Registrable Securities for resale by Holders in the manner or manners designated
by them (including, without limitation, one or more underwritten offerings). The
Company shall not permit any securities other than the Registrable Securities to
be included in the Initial Shelf Registration or any Subsequent Shelf
Registration (as defined below).

              The Company shall use its reasonable best efforts to cause the
Initial Shelf Registration to be declared effective under the Securities Act on
or prior to the Effectiveness Date and to keep such Initial Shelf Registration
continuously effective under the Securities Act until the date that is two years
from the Closing Date (as it may be shortened pursuant to clause (i) or clause
(ii) immediately following, the "EFFECTIVENESS PERIOD"), or such shorter period
ending when (i) all the shares of Registrable Securities covered by the Initial
Shelf



                                       -4-
<PAGE>

Registration have been sold in the manner set forth and as contemplated in the
Initial Shelf Registration, (ii) the date on which all the Registrable
Securities (x) held by Persons who are not affiliates of the Company may be
resold pursuant to Rule 144(k) under the Securities Act or (y) cease to be
outstanding, or (iii) a Subsequent Shelf Registration covering all of the
Registrable Securities has been declared effective under the Securities Act.

              (b) SUBSEQUENT SHELF REGISTRATIONS. If the Initial Shelf
Registration or any Subsequent Shelf Registration ceases to be effective for any
reason at any time during the Effectiveness Period (other than because of the
sale of all of the securities registered thereunder), the Company shall use its
reasonable best efforts to obtain the prompt withdrawal of any order suspending
the effectiveness thereof, and in any event shall within 45 days of such
cessation of effectiveness amend the Initial Shelf Registration in a manner to
obtain the withdrawal of the order suspending the effectiveness thereof, or file
an additional "shelf" Registration Statement pursuant to Rule 415 covering all
of the Registrable Securities (a "SUBSEQUENT SHELF REGISTRATION"). If a
Subsequent Shelf Registration is filed, the Company shall use its reasonable
best efforts to cause the Subsequent Shelf Registration to be declared effective
under the Securities Act as soon as practicable after such filing and to keep
such Registration Statement continuously effective for a period equal to the
number of days in the Effectiveness Period less the aggregate number of days
during which the Initial Shelf Registration or any Subsequent Shelf Registration
was previously continuously effective. As used herein the term "SHELF
REGISTRATION" means the Initial Shelf Registration and any Subsequent Shelf
Registration and the term "SHELF REGISTRATION STATEMENT" means any Registration
Statement filed in connection with a Shelf Registration.

              (c) SUPPLEMENTS AND AMENDMENTS. The Company shall promptly
supplement and amend the Shelf Registration if required by the rules,
regulations or instructions applicable to the registration form used for such
Shelf Registration, if required by the Securities Act, or if reasonably
requested by the Holders of the majority in Amount of Registrable Securities
covered by such Registration Statement or by any underwriter of such Registrable
Securities.

3.       LIQUIDATED DAMAGES

              (a) The Company and the Initial Purchasers agree that the Holders
of Registrable Securities will suffer damages if the Company fails to fulfill
its obligations under Section 2 hereof and that it would not be feasible to
ascertain the extent of such damages with precision. Accordingly, the Company
agrees to pay liquidated damages on the Registrable Securities ("LIQUIDATED
DAMAGES") under the circumstances and to the extent set forth below (each of
which shall be given independent effect; each a "REGISTRATION DEFAULT"):

              (i) if the Initial Shelf Registration is not filed on or prior to

         the Filing Date, then commencing on the day after the Filing Date,
         Liquidated Damages shall accrue on the Registrable Securities at a
         rate of 0.50% per annum on the Amount of Registrable Securities
         for the first 90 days immediately following the Filing Date, such
         Liquidated

                                      -5-

<PAGE>

         Damages increasing by an additional 0.50% per annum at the beginning of
         each subsequent 90-day period;

              (ii) if the Initial Shelf Registration is not declared
         effective by the SEC on or prior to the Effectiveness Date, then
         commencing on day after the Effectiveness Date, Liquidated Damages
         shall accrue on the Registrable Securities at a rate of 0.50% per annum
         on the Amount of Registrable Securities for the first 90 days
         immediately following the day after such Effectiveness Date, such
         Liquidated Damages increasing by an additional 0.50% per annum at the
         beginning of each subsequent 90-day period; and

              (iii) if a Shelf Registration has been declared effective and
         such Shelf Registration ceases to be effective at any time during the
         Effectiveness Period (other than as permitted under Section 3(b)), then
         Liquidated Damages shall accrue on the Registrable Securities at a rate
         of 0.50% per annum on the Amount of Registrable Securities for the
         first 90 days commencing on the day such Shelf Registration ceases to
         be effective, such Liquidated Damages increasing by an additional 0.50%
         per annum at the beginning of each such subsequent 90-day period;

PROVIDED, HOWEVER, that Liquidated Damages on the Registrable Securities may not
accrue under more than one of the foregoing clauses (i), (ii) or (iii) at any
one time and at no time shall the aggregate amount of Liquidated Damages
accruing exceed in the aggregate 1.0% per annum of the Amount of Registrable
Securities; PROVIDED, FURTHER, HOWEVER, that (1) upon the filing of the Shelf
Registration as required hereunder (in the case of clause (a)(i) of this Section
3), (2) upon the effectiveness of the Shelf Registration as required hereunder
(in the case of clause (a)(ii) of this Section 3), or (3) upon the effectiveness
of a Shelf Registration which had ceased to remain effective (in the case of
(a)(iii) of this Section 3), Liquidated Damages on the Registrable Securities as
a result of such clause (or the relevant subclause thereof), as the case may be,
shall cease to accrue. It is understood and agreed that, notwithstanding any
provision to the contrary, so long as any Registrable Security is then covered
by an effective Shelf Registration Statement, no Liquidated Damages shall accrue
on such Registrable Security.

              (b) Notwithstanding paragraph (a) of this Section 3, the
Company shall be permitted to suspend the effectiveness of a Shelf Registration
for up to 30 consecutive days in any 90 day period, for a total of not more than
60 days in any 365 day period, without paying Liquidated Damages.

              (c) So long as Convertible Notes remain outstanding, the
Company shall notify the Trustee within two Business Days after each and every
date on which an event occurs in respect of which Liquidated Damages is required
to be paid. Any amounts of Liquidated Damages due pursuant to (a)(i), (a)(ii) or
(a)(iii) of this Section 3 will be payable in cash semi-annually on each March 1
and September 15 (each a "DAMAGES PAYMENT Date"), commencing with the first such
date occurring after any such Liquidated Damages commences to




                                      -6-
<PAGE>

accrue, to Holders to whom regular interest is payable on such Damages Payment
Date with respect to Convertible Notes that are Registrable Securities and to
Persons that are registered Holders 15 days prior to such Damages Payment Date
with respect to Underlying Shares that are Registrable Securities. The amount of
Liquidated Damages for Registrable Securities will be determined by multiplying
the applicable rate of Liquidated Damages by the Amount of Registrable
Securities outstanding on the Damages Payment Date following such Registration
Default in the case of the first such payment of Liquidated Damages with respect
to a Registration Default (and thereafter at the next succeeding Damages Payment
Date until the cure of such Registration Default), multiplied by a fraction, the
numerator of which is the number of days such Liquidated Damages rate was
applicable during such period (determined on the basis of a 360-day year
comprised of twelve 30-day months and, in the case of a partial month, the
actual number of days elapsed), and the denominator of which is 360.

4.       REGISTRATION PROCEDURES

              In connection with the filing of any Registration Statement
pursuant to Section 2 hereof, the Company shall effect such registrations to
permit the sale of the securities covered thereby in accordance with the
intended method or methods of disposition thereof, and pursuant thereto and in
connection with any Registration Statement filed by the Company hereunder the
Company shall:

              (a) Prepare and file with the SEC prior to the Filing Date, a
Registration Statement or Registration Statements as prescribed by Section 2
hereof, and use its reasonable best efforts to cause each such Registration
Statement to become effective and remain effective as provided herein; PROVIDED,
HOWEVER, that before filing any Registration Statement or Prospectus or any
amendments or supplements thereto, the Company shall furnish to and afford the
Holders of the Registrable Securities covered by such Registration Statement,
their counsel and the managing underwriters, if any, a reasonable opportunity to
review copies of all such documents proposed to be filed (in each case, where
possible, at least five Business Days prior to such filing, or such later date
as is reasonable under the circumstances). The Company shall not file any
Registration Statement or Prospectus or any amendments or supplements thereto if
the Holders of a majority in Amount of Registrable Securities covered by such
Registration Statement, their counsel or the managing underwriters, if any,
shall reasonably object.

              (b) Prepare and file with the SEC such amendments and
post-effective amendments to each Shelf Registration, as may be necessary to
keep such Registration Statement continuously effective for the Effectiveness
Period; cause the related Prospectus to be supplemented by any Prospectus
supplement required by applicable law, and as so supplemented to be filed
pursuant to Rule 424 (or any similar provisions then in force) promulgated under
the Securities Act; and comply with the provisions of the Securities Act and the
Exchange Act applicable to it with respect to the disposition of all securities
covered by such Registration Statement as so amended or in such Prospectus as so
supplemented. The Com-




                                      -7-
<PAGE>

pany shall be deemed not to have used its reasonable best efforts to keep a
Registration Statement effective during the Effectiveness Period if it
voluntarily takes any action that would result in selling Holders of the
Registrable Securities covered thereby not being able to sell such Registrable
Securities during that period unless such action is required by applicable law
or unless the Company complies with this Agreement, including without limitation
the provisions of Section 4(k) hereof.

              (c) Notify the selling Holders of shares of Registrable
Securities, their counsel and the managing underwriters, if any, promptly (but
in any event within two Business Days) and confirm such notice in writing, (i)
when a Prospectus or any prospectus supplement or post-effective amendment has
been filed, and, with respect to a Registration Statement or any post-effective
amendment, when the same has become effective under the Securities Act
(including in such notice a written statement that any Holder may, upon request,
obtain, at the sole expense of the Company, one conformed copy of such
Registration Statement or post-effective amendment including financial
statements and schedules, documents incorporated or deemed to be incorporated by
reference and exhibits), (ii) of the issuance by the SEC of any stop order
suspending the effectiveness of a Registration Statement or of any order
preventing or suspending the use of any preliminary prospectus or the initiation
of any proceedings for that purpose, (iii) of the happening of any event, the
existence of any condition or any information becoming known that makes any
statement made in such Registration Statement or related Prospectus or any
document incorporated or deemed to be incorporated therein by reference untrue
in any material respect or that requires the making of any changes in or
amendments or supplements to such Registration Statement, Prospectus or
documents so that, in the case of the Registration Statement, it will not
contain any untrue statement of a material fact or omit to state any material
fact required to be stated therein or necessary to make the statements therein
not misleading, and that in the case of the Prospectus, it will not contain any
untrue statement of a material fact or omit to state any material fact required
to be stated therein or necessary to make the statements therein, in the light
of the circumstances under which they were made, not misleading and (iv) of the
Company's determination that a post-effective amendment to a Registration
Statement would be appropriate.

              (d) Use its reasonable best efforts to prevent the issuance of any
order suspending the effectiveness of a Registration Statement or of any order
preventing or suspending the use of a Prospectus and, if any such order is
issued, to use its reasonable best efforts to obtain the withdrawal of any such
order at the earliest possible moment.

              (e) If requested by the managing underwriter or underwriters (if
any) or the Holders of the majority in Amount of Registrable Securities being
sold in connection with an underwritten offering (i) promptly incorporate in a
prospectus supplement or post-effective amendment such information as the
managing underwriter or underwriters (if any), such Holders or counsel for any
of them reasonably determine is necessary to be included therein, (ii) make all
required filings of such prospectus supplement or such post-effective amendment
as soon as reasonably practicable after the Company has received notification of
the matters to



                                      -8-
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be incorporated in such prospectus supplement or post-effective amendment and
(iii) supplement or make amendments to such Registration Statement.

              (f) Furnish to each selling Holder of Registrable Securities, a
single counsel to such Holders (chosen in accordance with Section 5(b)) and each
managing underwriter, if any, at the sole expense of the Company, one conformed
copy of the Registration Statement or Registration Statements and each
post-effective amendment thereto, including financial statements and schedules,
and, if requested, all documents incorporated or deemed to be incorporated
therein by reference and all exhibits.

              (g) Deliver to each selling Holder of Registrable Securities, a
single counsel to such Holders (chosen in accordance with Section 5(b)) and the
underwriters, if any, at the sole expense of the Company, as many copies of the
Prospectus (including each form of preliminary prospectus) and each amendment or
supplement thereto and any documents incorporated by reference therein as such
Persons may reasonably request; and, subject to the second paragraph of Section
4(s) hereof, the Company hereby consents to the use of such Prospectus and each
amendment or supplement thereto by each of the selling Holders of Registrable
Securities and the underwriters or agents, if any, and dealers (if any), in
connection with the offering and sale of the Registrable Securities covered by
such Prospectus and any amendment or supplement thereto.

              (h) Prior to any public offering of Registrable Securities, to use
its reasonable best efforts to register or qualify, to the extent required by
applicable law, and to cooperate with the selling Holders of Registrable
Securities, the managing underwriter or underwriters, if any, and their
respective counsel in connection with the registration or qualification (or
exemption from such registration or qualification) of such Registrable
Securities or offer and sale under the securities or Blue Sky laws of such
jurisdictions within the United States as any selling Holder, or the managing
underwriter or underwriters, if any, reasonably request; PROVIDED, HOWEVER, that
where Registrable Securities are offered other than through an underwritten
offering, the Company agrees to cause the Company's counsel to perform Blue Sky
investigations and file registrations and qualifications required to be filed
pursuant to this Section 4(h); keep each such registration or qualification (or
exemption therefrom) effective during the period such Registration Statement is
required to be kept effective and do any and all other acts or things reasonably
necessary or advisable to enable the disposition in such jurisdictions of the
Registrable Securities covered by the applicable Registration Statement;
PROVIDED, HOWEVER, that the Company shall not be required to (A) qualify
generally to do business in any jurisdiction where it is not then so qualified,
(B) take any action that would subject it to general service of process in any
such where it is not then so subject or (C) subject itself to taxation in excess
of a nominal dollar amount in any such jurisdiction where it is not then so
subject.

              (i) Cooperate with the selling Holders of Registrable Securities
and the managing underwriter or underwriters, if any, to facilitate the timely
preparation and delivery



                                      -9-
<PAGE>

of certificates representing shares of Registrable Securities to be sold, which
certificates shall not bear any restrictive legends and shall be in a form
eligible for deposit with The Depository Trust Company; and enable such shares
of Registrable Securities to be in such denominations and registered in such
names as the managing underwriter or underwriters, if any, or Holders may
reasonably request.

              (j) Use its reasonable best efforts to cause the Registrable
Securities covered by any Shelf Registration Statement to be registered with or
approved by such other governmental agencies or authorities as may be reasonably
necessary to enable the seller or sellers thereof or the underwriter or
underwriters, if any, to consummate the disposition of such Registrable
Securities, except as may be required solely as a consequence of the nature of
such selling Holder's business, in which case the Company will cooperate in all
reasonable respects with the filing of such Registration Statement and the
granting of such approvals.

              (k) Upon the occurrence of any event contemplated by paragraph
4(c)(ii), 4(c)(iii) or 4(c)(iv) hereof, as promptly as practicable prepare and
(subject to Section 4(a) hereof) file with the SEC, at the sole expense of the
Company, a supplement or post-effective amendment to the Registration Statement
or a supplement to the related Prospectus or any document incorporated or deemed
to be incorporated therein by reference, or file any other required document so
that, as thereafter delivered to the purchasers of the Registrable Securities
being sold thereunder, any such Prospectus will not contain an untrue statement
of a material fact or omit to state a material fact required to be stated
therein or necessary to make the statements therein, in the light of the
circumstances under which they were made, not misleading.

              (l) Prior to the effective date of the first Registration
Statement relating to the Registrable Securities, (i) provide the Trustee with
certificates for the Registrable Securities in a form eligible for deposit with
The Depository Trust Company and (ii) provide a CUSIP number for the Registrable
Securities.

              (m) In connection with any underwritten offering of Registrable
Securities pursuant to a Shelf Registration, enter into an underwriting
agreement as is customary in underwritten offerings of securities similar to the
Registrable Securities and take all such other actions as are reasonably
requested by the managing underwriter or underwriters in order to expedite or
facilitate the registration or the disposition of such Registrable Securities
and, in such connection, (i) make such representations and warranties to, and
covenants with, the underwriters with respect to the business of the Company and
its subsidiaries (including any acquired business, properties or entity, if
applicable) and the Registration Statement, Prospectus and documents, if any,
incorporated or deemed to be incorporated by reference therein, in each case, as
are customarily made by issuers to underwriters in underwritten offerings of
securities similar to the Registrable Securities and confirm the same in writing
if and when requested; (ii) obtain the written opinion of counsel to the Company
and written updates thereof in form, scope and substance reasonably satisfactory
to the managing underwriter or underwriters, ad-



                                      -10-
<PAGE>

dressed to the underwriters covering the matters customarily covered in opinions
requested in underwritten offerings of securities similar to the Registrable
Securities and such other matters as may be reasonably requested by the managing
underwriter or underwriters; and (iii) obtain "cold comfort" letters and updates
thereof in form, scope and substance reasonably satisfactory to the managing
underwriter or underwriters from the independent certified public accountants of
the Company (and, if necessary, any other independent certified public
accountants of any subsidiary of the Company or of any business acquired by the
Company for which financial statements and financial data are, or are required
to be, included or incorporated by reference in the Registration Statement),
addressed to each of the underwriters, such letters to be in customary form and
covering matters of the type customarily covered in "cold comfort" letters in
connection with underwritten offerings of securities similar to the Registrable
Securities and such other matters as reasonably requested by the managing
underwriter or underwriters as permitted by the Statement on Auditing Standards
No. 72. The above shall be done as and to the extent required by such
underwriting agreement.

              (n) Make available for inspection by any selling Holder of such
Registrable Securities being sold, any underwriter participating in any such
disposition of Registrable Securities, if any, and any attorney, accountant or
other agent retained by any such selling Holder, or underwriter (collectively,
the "Inspectors"), at the offices where normally kept, during reasonable
business hours at such time or times as shall be mutually convenient for the
Company and the Inspectors as a group, all financial and other records,
pertinent corporate documents and instruments of the Company and its
subsidiaries (collectively, the "Records") as shall be reasonably necessary to
enable them to exercise any applicable due diligence responsibilities, and cause
the officers, directors and employees of the Company and its subsidiaries to
supply all information reasonably requested by any such Inspector in connection
with such Registration Statement. Records that the Company determines, in good
faith, to be confidential and any Records that it notifies the Inspectors are
confidential shall not be disclosed by any Inspector unless (i) the disclosure
of such Records is necessary to avoid or correct a material misstatement or
material omission in such Registration Statement, (ii) the release of such
Records is ordered pursuant to a subpoena or other order from a court of
competent jurisdiction, (iii) disclosure of such information is, in the opinion
of counsel for any Inspector, necessary or advisable in connection with any
action, claim, suit or proceeding, directly, involving or potentially involving
such Inspector and arising out of, based upon, relating to, or involving this
Agreement or any transactions contemplated hereby or arising hereunder or (iv)
the information in such Records has been made generally available to the public
other than through the acts of such Inspector; PROVIDED, HOWEVER, that prior
notice shall be provided as soon as practicable to the Company of the potential
disclosure of any information by such Inspector pursuant to clauses (ii) or
(iii) of this sentence to permit the Company to obtain a protective order (or
waive the provisions of this paragraph (n)) and that such Inspector shall take
such actions as are reasonably necessary to protect the confidentiality of such
information (if practicable) to the extent such action is otherwise not
inconsistent with, an impairment of or in derogation of the rights and interests
of the Holder or any Inspector, or (iv) the information in



                                      -11-
<PAGE>

such Records has been made generally available to the public other than as a
result of a breach of this Agreement.

              (o) Provide (i) the Holders of the Registrable Securities to be
included in such Registration Statement and not more than one counsel for all
the Holders of such Registrable Securities, (ii) the underwriters (which term,
for purposes of this Registration Rights Agreement, shall include a Person
deemed to be an underwriter within the meaning of Section 2(11) of the
Securities Act), if any, thereof, (iii) the sales or placement agent, if any,
thereof, and (D) one counsel for such underwriters or agents, reasonable
opportunity to participate in the preparation of such Registration Statement,
each prospectus included therein or filed with the SEC, and each amendment or
supplement thereto.

              (p) Comply with all applicable rules and regulations of the SEC
and make generally available to its securityholders earning statements
satisfying the provisions of Section 11(a) of the Securities Act and Rule 158
thereunder (or any similar rule promulgated under the Securities Act) no later
than 45 days after the end of any 12-month period (or 90 days after the end of
any 12-month period if such period is a fiscal year) (i) commencing at the end
of any fiscal quarter in which Registrable Securities are sold to underwriters
in a firm commitment or best efforts underwritten offering and (ii) if not sold
to underwriters in such an offering, commencing on the first day of the first
fiscal quarter of the Company after the effective date of a Registration
Statement, which statements shall cover said 12-month periods.

              (q) Cooperate with each seller of Registrable Securities covered
by any Registration Statement and each underwriter, if any, participating in the
disposition of such Registrable Securities and their respective counsel in
connection with any filings required to be made with the National Association of
Securities Dealers, Inc. (the "NASD"), including, if the Conduct Rules of the
NASD or any successor thereto as amended from time to time so require, engaging
a "qualified independent underwriter" ("QIU") as contemplated therein and making
Records available to such QIU as though it were a participating underwriter for
the purposes of Section 4(n) and otherwise applying the provisions of this
Agreement to such QIU (including indemnification) as though it were a
participating underwriter.

              (r) Cause the Indenture to be qualified under the TIA not later
than the effective date of the first Registration Statement relating to the
Registrable Securities; and in connection therewith, cooperate with the Trustee
and the Holders of the Registrable Securities to effect such changes to the
Indenture as may be required for the Indenture to be so qualified in accordance
with the terms of the TIA; and execute, and use its reasonable best efforts to
cause the Trustee to execute, all documents as may be required to effect such
changes and all other forms and documents required to be filed with the SEC to
enable the Indenture to be so qualified in a timely manner.

                                      -12-
<PAGE>

              (s) Use its reasonable best efforts to take all other steps
necessary or advisable to effect the registration of the Registrable Securities
covered by a Registration Statement contemplated hereby.

              The Company may require each seller of Registrable Securities as
to which any registration is being effected to furnish to the Company such
information regarding such seller and the distribution of such Registrable
Securities as the Company may, from time to time, reasonably request to the
extent necessary or advisable to comply with the Securities Act. The Company may
exclude from such registration the Registrable Securities of any seller if such
seller fails to furnish such information within 20 Business Days after receiving
such request. Each seller as to which any Shelf Registration is being effected
agrees to furnish promptly to the Company all information required to be
disclosed so that the information previously furnished to the Company by such
seller is not materially misleading and does not omit to state any material fact
required to be stated therein or necessary to make the statements therein not
misleading in light of the circumstances under which they were made.

              Each Holder of Registrable Securities agrees by acquisition of
such Registrable Securities that, upon actual receipt of any notice from the
Company of the happening of any event of the kind described in Section 4(c)(ii),
4(c)(iii) or 4(c)(iv) hereof, such Holder will forthwith discontinue disposition
of such Registrable Securities covered by such Registration Statement or
Prospectus until such Holder's receipt of the copies of the supplemented or
amended Prospectus contemplated by Section 4(k) hereof, or until it is advised
in writing by the Company that the use of the applicable Prospectus may be
resumed, and has received copies of any amendments or supplements thereto.

5.       REGISTRATION EXPENSES

              (a) All fees and expenses incident to the performance of or
compliance with this Agreement by the Company shall be borne by the Company,
including, without limitation, (i) all registration and filing fees (including,
without limitation, (A) fees with respect to filings required to be made with
the NASD in connection with an underwritten offering and (B) fees and expenses
of compliance with state securities or Blue Sky laws (including, without
limitation, reasonable fees and disbursements of counsel in connection with Blue
Sky qualifications of the Registrable Securities and determination of the
eligibility of the Registrable Securities for investment under the laws of such
jurisdictions as provided in Section 4(h) hereof), (ii) printing expenses,
including, without limitation, expenses of printing certificates for Registrable
Securities in a form eligible for deposit with The Depository Trust Company and
of printing prospectuses if the printing of prospectuses is requested by the
managing underwriter or underwriters, if any, or by the Holders of the majority
in Amount of Registrable Securities included in any Registration Statement,
(iii) messenger, telephone and delivery expenses, (iv) fees and disbursements of
counsel for the Company and reasonable fees and disbursements of special counsel
for the sellers of Registrable Securities (subject to the provisions of Section
5(b) hereof), (v) fees and disbursements of all independent certified public



                                      -13-
<PAGE>

accountants referred to in Section 4(m)(iii) hereof (including, without
limitation, the expenses of any special audit and "cold comfort" letters
required by or incident to such performance), (vi) Securities Act liability
insurance, if the Company desires such insurance, (vii) fees and expenses of all
other Persons retained by the Company, (viii) internal expenses of the Company
(including, without limitation, all salaries and expenses of officers and
employees of the Company performing legal or accounting duties), (ix) the
expense of any annual audit, (x) the fees and expenses incurred in connection
with the listing of the securities to be registered on any securities exchange,
if applicable, and (xi) the expenses relating to printing, word processing and
distributing all Registration Statements, underwriting agreements, securities
sales agreements and any other documents necessary in order to comply with this
Agreement. Nothwithstanding anything in this Agreement to the contrary, each
Holder shall pay all underwriting discounts and brokerage commissions with
respect to any Registrable Securities sold by it.

              (b) The Company shall reimburse the Holders of the Registrable
Securities being registered in a Shelf Registration for the reasonable fees and
disbursements of not more than one counsel chosen by the Holders of a majority
in Amount of Registrable Securities to be included in such Registration
Statement.

6.       INDEMNIFICATION

              The Company agrees to indemnify and hold harmless (i) each Initial
Purchaser, (ii) each Holder, (iii) each Person, if any, who controls (within the
meaning of either Section 15 of the Securities Act or Section 20 of the Exchange
Act) any of the foregoing (any of the Persons referred to in this clause (iii)
being hereinafter referred to as a "controlling person"), (iv) the respective
officers, directors, partners, employees, representatives and agents of the
Initial Purchasers, the Holders (including predecessor Holders) or any
controlling person (any person referred to in clause (i), (ii), (iii) or (iv)
may hereinafter be referred to as an "INDEMNIFIED HOLDER"), from and against any
and all losses, claims, damages, liabilities and judgments (including, without
limitation, reasonable legal fees and other expenses incurred in connection with
any suit, action or proceeding or any claim asserted) caused by any untrue
statement or alleged untrue statement of a material fact contained in any
Registration Statement or Prospectus, or any amendment or supplement thereto or
any related preliminary prospectus, or caused by any omission or alleged
omission to state therein a material fact required to be stated therein or
necessary to make the statements therein not misleading, except insofar as such
losses, claims, damages or liabilities are caused by any untrue statement or
omission or alleged untrue statement or omission made in reliance upon and in
conformity with information relating to any Holder furnished to the Company in
writing by such Holder expressly for use in therein; PROVIDED, HOWEVER, that the
Company shall not be liable to any Indemnified Holder under the indemnity
agreement of this paragraph with respect to any preliminary prospectus to the
extent that any such loss, claim, damage, liability, judgment or expense of such
Indemnified Holder results from the fact that such Indemnified Holder sold
Registrable Securities under a Registration Statement to a Person as to whom it
shall be established that there



                                      -14-
<PAGE>

was not sent or given, at or prior to the written confirmation of such sale, a
copy of the Prospectus (or of the preliminary prospectus as then amended or
supplemented if the Company shall have furnished such Indemnified Holder with
such amendment or supplement thereto on a timely basis), in any case where such
delivery is required by applicable law and the loss, claim, damage, liability or
expense of such Indemnified Holder results from an untrue statement or omission
of a material fact contained in the preliminary prospectus which was corrected
in the Prospectus (or in the preliminary prospectus as then amended or
supplemented if the Company shall have furnished such Indemnified Holder with
such amendment or supplement thereto, as the case may be, on a timely basis).
The Company shall notify Indemnified Holder promptly of the institution, threat
or assertion of any claim, proceeding (including any governmental investigation)
or litigation in connection with the matters addressed by this Agreement which
involves the Company or such Indemnified Holder.

              Each Holder agrees, severally and not jointly, to indemnify and
hold harmless the Company, its directors, officers and each Person who controls
the Company within the meaning of either Section 15 of the Securities Act or
Section 20 of the Exchange Act to the same extent as the foregoing indemnity
from the Company to each Holder, but only with reference to such losses, claims,
damages or liabilities which are caused by any untrue statement or omission or
alleged untrue statement or omission made in reliance upon and in conformity
with information relating to a Holder furnished to the Company in writing by
such Holder expressly for use in any Registration Statement or Prospectus, or
any amendment or supplement thereto or any related preliminary prospectus.

              If any suit, action, proceeding (including any governmental or
regulatory investigation), claim or demand shall be brought or asserted against
any Person in respect of which indemnity may be sought pursuant to either of the
two preceding paragraphs, such Person (the "INDEMNIFIED PERSON") shall promptly
notify the Person or Persons against whom such indemnity may be sought (each an
"INDEMNIFYING PERSON") in writing, and such Indemnifying Person, upon request of
the Indemnified Person, shall retain counsel reasonably satisfactory to the
Indemnified Person to represent the Indemnified Person and any others entitled
to indemnification pursuant to this Section 6 that the Indemnifying Person may
designate in such proceeding and shall pay the fees and expenses of such counsel
related to such proceeding. In any such proceeding, any Indemnified Person shall
have the right to retain its own counsel, but the fees and expenses of such
counsel shall be at the expense of such Indemnified Person unless (i) such
Indemnifying Person and the Indemnified Person shall have mutually agreed to the
contrary, (ii) such Indemnifying Person has failed within a reasonable time to
retain counsel reasonably satisfactory to such Indemnified Person or (iii) the
named parties in any such proceeding (including any impleaded parties) include
an Indemnifying Person and an Indemnified Person and representation of both
parties by the same counsel would be inappropriate due to actual or potential
differing interests between them. It is understood that an Indemnifying Person
shall not, in connection with any proceeding or related proceedings in the same
jurisdiction, be liable for the fees and expenses of more than one separate firm
(in addition to any local counsel) for all Indemnified Persons, and that all
such fees and expenses shall



                                      -15-
<PAGE>

be reimbursed as they are incurred. Any such separate firm for the Indemnified
Holders shall be designated in writing by the Holders of the majority in Amount
of Registrable Securities, and any such separate firm for the Company, its
directors, respective officers and such control Persons of the Company shall be
designated in writing by the Company. The Indemnifying Person shall not be
liable for any settlement of any proceeding effected without its written
consent, but if settled with such consent or if there be a final judgment for
the plaintiff, such Indemnifying Person agrees to indemnify any Indemnified
Person from and against any loss or liability by reason of such settlement or
judgment. No Indemnifying Person shall, without the prior written consent of the
Indemnified Person, effect any settlement of any pending or threatened
proceeding in respect of which any Indemnified Person is or could have been a
party and indemnity could have been sought hereunder by such Indemnified Person,
unless such settlement includes an unconditional release of such Indemnified
Person from all liability on claims that are the subject matter of such
proceeding.

              If the indemnification provided for in the first and second
paragraphs of this Section 6 is unavailable to an Indemnified Person or
insufficient in respect of any losses, claims, damages or liabilities referred
to therein, then each Indemnifying Person under such paragraph, in lieu of
indemnifying such Indemnified Person thereunder, shall contribute to the amount
paid or payable by such Indemnified Person as a result of such losses, claims,
damages or liabilities (i) in such proportion as is appropriate to reflect the
relative benefits received by the Indemnifying Person on the one hand and the
Indemnified Person on the other hand pursuant to the Purchase Agreement or from
the offering of the Registrable Securities pursuant to any Shelf Registration or
(ii) if the allocation provided by clause (i) above is not permitted by
applicable law, in such proportion as is appropriate to reflect not only the
relative benefits referred to in clause (i) above but also the relative fault of
the Indemnifying Person on the one hand and the Indemnified Person on the other
in connection with the statements or omissions that resulted in such losses,
claims, damages or liabilities, as well as any other relevant equitable
considerations. The relative benefits received by the Company on the one hand
and any Indemnified Holder on the other shall be deemed to be in the same
proportion as the total net proceeds from the initial offering and sale of
Convertible Notes (before deducting expenses) received by the Company bear to
the total net proceeds received by such Indemnified Holder from sales of
Registrable Securities giving rise to such obligations. The relative fault of
the parties shall be determined by reference to, among other things, whether the
untrue or alleged untrue statement of a material fact or the omission or alleged
omission to state a material fact relates to information supplied by the Company
or such Indemnified Holder and the parties' relative intent, knowledge, access
to information and opportunity to correct or prevent such statement or omission.

              Each of the Company and the Initial Purchasers agrees that it
would not be just and equitable if contribution pursuant to this Section 6 were
determined by PRO RATA allocation or by any other method of allocation that does
not take account of the equitable considerations referred to in the



                                      -16-
<PAGE>

immediately preceding paragraph. The amount paid or payable by an Indemnified
Person as a result of the losses, claims, damages and liabilities referred to in
the immediately preceding paragraph shall be deemed to include, subject to the
limitations set forth above, any legal or other expenses incurred by such
Indemnified Person in connection with investigating or defending any such action
or claim. Notwithstanding the provisions of this Section 6, in no event shall
any Holder be required to contribute any amount in excess of the amount by which
the net proceeds received by such Holder from the sale of the Registrable
Securities pursuant to a Shelf Registration Statement exceeds the amount of
damages which such Holder would have otherwise been required to pay by reason of
such untrue or alleged untrue statement or omission or alleged omission. No
Person guilty of fraudulent misrepresentation (within the meaning of Section
11(F) of the Securities Act) shall be entitled to contribution from any Person
who was not guilty of such fraudulent misrepresentation.

              The remedies provided for in this Section 6 are not exclusive and
shall not limit any rights or remedies that may otherwise be available to any
indemnified party at law or in equity.

              The indemnity and contribution agreements contained in this
Section 6 shall remain operative and in full force and effect regardless of (i)
any termination of this Agreement, (ii) any investigation made by or on behalf
of any Holder or any Person controlling any Holder or by or on behalf of the
Company, its officers or directors or any other Person controlling any of the
Company and (iii) acceptance of and payment for any of the Registrable
Securities.

7. RULES 144 AND 144A.

              The Company covenants that it will file the reports required to be
filed by it under the Securities Act and the Exchange Act and the rules and
regulations adopted by the SEC thereunder in a timely manner in accordance with
the requirements of the Securities Act and the Exchange Act and, for so long as
any Registrable Securities remain outstanding, if at any time the Company is not
required to file such reports, it will, upon the request of any Holder or
beneficial owner of Registrable Securities, make available such information
necessary to permit sales pursuant to Rule 144A under the Securities Act. The
Company further covenants that, for so long as any Registrable Securities remain
outstanding, it will use its reasonable best efforts to take such further action
as any Holder of Registrable Securities may reasonably request, all to the
extent required from time to time to enable such holder to sell Registrable
Securities without registration under the Securities Act within the limitation
of the exemptions provided by (a) Rule 144(k) and Rule 144A under the Securities
Act, as such rules may be amended from time to time, or (b) any similar rule or
regulation hereafter adopted by the SEC. Notwithstanding the foregoing, nothing
in this Section 7 shall be deemed to require the Company to register any of its
securities pursuant to the Exchange Act.


                                      -17-
<PAGE>

8.       UNDERWRITTEN REGISTRATIONS.

              If any of the Registrable Securities covered by any Shelf
Registration are to be sold in an underwritten offering, the investment banker
or investment bankers and manager or managers that will manage the offering will
be selected by the Holders of the majority in Amount of Registrable Securities
to be included in such offering and will be reasonably acceptable to the
Company.

              No Holder of Registrable Securities may participate in any
underwritten registration hereunder unless such Holder (a) agrees to sell such
Holder's Registrable Securities on the basis provided in any underwriting
arrangements approved by the Persons entitled hereunder to approve such
arrangements and (b) completes and executes all questionnaires, powers of
attorney, indemnities, underwriting agreements and other documents reasonably
required under the terms of such underwriting arrangements.

9.       MISCELLANEOUS

              (a) NO INCONSISTENT AGREEMENTS. The Company has not, as of the
date hereof, and the Company shall not, after the date of this Agreement, enter
into any agreement with respect to any of its securities that is inconsistent
with the rights granted to the Holders of Registrable Securities in this
Agreement or otherwise conflicts with the provisions hereof. The Company has not
entered and will not enter into any agreement with respect to any of its
securities that will grant to any Person piggyback registration rights with
respect to a Registration Statement, except to the extent any existing right has
heretofore been waived.

              (b) ADJUSTMENTS AFFECTING REGISTRABLE SECURITIES. The Company
shall not, directly or indirectly, take any action with respect to the
Registrable Securities as a class that would adversely affect the ability of the
Holders of Registrable Securities to include such Registrable Securities in a
registration undertaken pursuant to this Agreement.

              (c) AMENDMENTS AND WAIVERS. The provisions of this Agreement may
not be amended, modified or supplemented, and waivers or consents to departures
from the provisions hereof may not be given, otherwise than with the prior
written consent of the Company and the Holders of not less than the majority in
Amount of Registrable Securities; PROVIDED, HOWEVER, that Section 6 and this
Section 9(c) may not be amended, modified or supplemented without the prior
written consent of the Company and each Holder (including, in the case of an
amendment, modification or supplement of Section 6, any Person who was a Holder
of Registrable Securities disposed of pursuant to any Registration Statement).
Notwithstanding the foregoing, a waiver or consent to depart from the provisions
hereof with respect to a matter that relates exclusively to the rights of
Holders of Registrable Securities whose securities are being sold pursuant to a
Registration Statement and that does not directly or indirectly affect, impair,
limit or compromise the rights of other Holders of Registrable Securities may be




                                      -18-
<PAGE>

given by Holders of at least a majority in Amount of the Registrable Securities
being sold by such Holders pursuant to such Registration Statement.

        (d)    NOTICES. All notices and other communications (including
without limitation any notices or other communications to the Trustee) provided
for or permitted hereunder shall be made in writing by hand-delivery, registered
first-class mail, next-day air courier or facsimile:

              (1) if to a Holder of the Registrable Securities, at the most
current address of such Holder set forth on the records of the registrar under
the Indenture, in the case of Holders of Convertible Notes, and the stock ledger
of the Company, in the case of Holders of common stock of the Company.

              (2) if to the Initial Purchasers:

              J.P. MORGAN SECURITIES INC.
              U.S. BANCORP PIPER JAFFRAY INC.
              CHASE SECURITIES INC.
              WARBURG DILLION READ LLC
              c/o J.P. Morgan Securities Inc.
              60 Wall Street
              New York, New York 10260
              Attention:        Syndicate Department

         with copies to:

              Cahill Gordon & Reindel
              80 Pine Street
              New York, New York 10005
              Facsimile No.:  (212) 269-5420
              Attention:        Daniel J. Zubkoff, Esq.

              (3) if to the Company, at the addresses as follows:

              Alexion Pharmaceuticals, Inc.
              25 Science Park, Suite 360
              New Haven, Connecticut 06511
              Facsimile No.:  (203) 776-2089
              Attention:        President

         with copies to:

              Fulbright & Jaworski L.L.P.
              666 Fifth Avenue, 31st Floor



                                      -19-
<PAGE>

              New York, New York 10103
              Facsimile No.:  (212) 318-3000
              Attention:        Lawrence A. Spector, Esq.

              All such notices and communications shall be deemed to have been
duly given: when delivered by hand, if personally delivered; five Business Days
after being deposited in the mail, postage prepaid, if mailed; one Business Day
after being timely delivered to a next-day air courier; and when receipt is
acknowledged by the addressee, if sent by facsimile.

              (e) SUCCESSORS AND ASSIGNS. This Agreement shall inure to the
benefit of and be binding upon the successors and assigns of each of the parties
hereto, including the Holders; PROVIDED, HOWEVER, that this Agreement shall not
inure to the benefit of or be binding upon a successor or assign of a Holder
unless and except to the extent such successor or assign holds Registrable
Securities.

              (f) COUNTERPARTS. This Agreement may be executed in any number of
counterparts and by the parties hereto in separate counterparts, each of which
when so executed shall be deemed to be an original and all of which taken
together shall constitute one and the same agreement.

              (g) HEADINGS. The headings in this Agreement are for convenience
of reference only and shall not limit or otherwise affect the meaning hereof.

              (h) GOVERNING LAW. THIS AGREEMENT SHALL BE GOVERNED BY AND
CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK, AS APPLIED TO
CONTRACTS MADE AND PERFORMED WHOLLY WITHIN THE STATE OF NEW YORK, WITHOUT REGARD
TO PRINCIPLES OF CONFLICTS OF LAW. EACH OF THE PARTIES HERETO AGREES TO SUBMIT
TO THE JURISDICTION OF THE FEDERAL AND NEW YORK STATE COURTS SITTING IN
MANHATTAN, NEW YORK CITY, THE STATE OF NEW YORK, IN ANY ACTION OR PROCEEDING
ARISING OUT OF OR RELATING TO THIS AGREEMENT.

              (i) SEVERABILITY. If any term, provision, covenant or restriction
of this Agreement is held by a court of competent jurisdiction to be invalid,
illegal, void or unenforceable, the remainder of the terms, provisions,
covenants and restrictions set forth herein shall remain in full force and
effect and shall in no way be affected, impaired or invalidated, and the parties
hereto shall use their best efforts to find and employ an alternative means to
achieve the same or substantially the same result as that contemplated by such
term, provision, covenant or restriction. It is hereby stipulated and declared
to be the intention of the parties that they would have executed the remaining
terms, provisions, covenants and restrictions without including any of such that
may be hereafter declared invalid, illegal, void or unenforceable.


                                      -20-
<PAGE>


              (j) SECURITIES HELD BY THE COMPANY OR ITS AFFILIATES. Whenever the
consent or approval of Holders of a specified percentage in Amount of
Registrable Securities is required hereunder, Registrable Securities held by the
Company or its affiliates (as such term is defined in Rule 405 under the
Securities Act) shall not be counted in determining whether such consent or
approval was given by the Holders of such required percentage.

              (k) THIRD PARTY BENEFICIARIES. Holders of Registrable Securities
are intended third party beneficiaries of this Agreement and this Agreement may
be enforced by such Persons.

              (l) ENTIRE AGREEMENT. This Agreement, together with the Purchase
Agreement and the Indenture, is intended by the parties as a final and exclusive
statement of the agreement and understanding of the parties hereto in respect of
the subject matter contained herein and therein and any and all prior oral or
written agreements, representations, or warranties, contracts, understandings,
correspondence, conversations and memoranda between the Initial Purchasers on
the one hand and the Company on the other, or between or among any agents,
representatives, parents, subsidiaries, affiliates, predecessors in interest or
successors in interest with respect to the subject matter hereof and thereof are
merged herein and replaced hereby.



                                      -21-
<PAGE>




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

                                       ALEXION PHARMACEUTICALS, INC.

                                       By: /s/ David W. Keiser
                                           -----------------------------------
                                           Name:  David W. Keiser
                                           Title: Executive Vice President
                                                  and Chief Operating Officer

                                       J.P. MORGAN SECURITIES INC.
                                       U.S. BANCORP PIPER JAFFRAY INC.
                                       CHASE SECURITIES INC.
                                       WARBURG DILLION READ LLC

                                       By:  J.P. MORGAN SECURITIES INC.

                                             By: /s/ John D. Fowler, Jr.
                                                 -------------------------------
                                                 Name:  John D. Fowler, Jr.
                                                 Title: Managing Director