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                                  $500,000,000

                               GENZYME CORPORATION

                 3% CONVERTIBLE SUBORDINATED DEBENTURES DUE 2021


                          REGISTRATION RIGHTS AGREEMENT


                                                                     May 3, 2001


Credit Suisse First Boston Corporation
Goldman, Sachs & Co.
Salomon Smith Barney Inc.
c/o Credit Suisse First Boston Corporation
Eleven Madison Avenue
New York, New York 10010-3629

Dear Sirs:

       Genzyme Corporation, a Massachusetts corporation (the "Company"),
proposes to issue and sell to Credit Suisse First Boston Corporation, Goldman,
Sachs & Co. and Salomon Smith Barney Inc. (the "Initial Purchasers"), upon the
terms set forth in a purchase agreement of even date herewith (the "Purchase
Agreement"), $500,000,000 aggregate principal amount of its 3% Convertible
Subordinated Debentures due 2021 (the "Debentures"). The Debentures will be
issued pursuant to an Indenture, dated as of May 8, 2001 (the "Indenture") by
and between the Company and State Street Bank and Trust Company (the "Trustee").
Under the terms of the Indenture, the Debentures are convertible, in whole or in
part, into shares of Genzyme General Division Common Stock, par value $.01 per
share (the "Conversion Shares" and, together with the Debentures, the
"Securities"), in accordance with the terms thereof at the conversion prices set
forth in the Debentures, as adjusted from time to time pursuant to the
Indenture. As an inducement to the Initial Purchasers, the Company agrees with
the Initial Purchasers, for the benefit of the holders of the Debentures
(including, without limitation, the Initial Purchasers) and Conversion Shares
(collectively, the "Holders"), as follows:

       1. RESALE SHELF REGISTRATION. (a) The Company shall, at its cost, use its
reasonable efforts to file (within 90 days after the Closing Date (as defined in
the Purchase Agreement)) with the Securities and Exchange Commission (the
"Commission") and thereafter shall use its reasonable efforts to cause to be
declared effective a registration statement (the "Resale Shelf Registration
Statement") on an appropriate form under the Securities Act of 1933, as amended
(the "Securities Act") relating to the offer and sale of the Transfer Restricted
Securities (as defined in Section 5 hereof) by the Holders thereof from time to
time in accordance with the methods of distribution set forth in the Resale
Shelf Registration Statement and Rule 415 under the Securities Act (hereinafter,
the "Resale Shelf Registration"); provided, however, that no Holder (other than
the Initial Purchasers) shall be entitled to have the Securities held by it
covered by such Resale Shelf Registration Statement unless such Holder agrees in
writing to be bound by all the provisions of this Agreement applicable to such
Holder.

       (b) The Company, subject to Section 2(h), shall use its reasonable
efforts to keep the Resale Shelf Registration Statement continuously effective
for a period of two years from the date of its effectiveness or

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such shorter period that will terminate when all the Securities covered by the
Resale Shelf Registration Statement (i) have been sold pursuant thereto or (ii)
are no longer Transfer Restricted Securities as defined in Section 5(d) (in any
such case, such period being called the "Shelf Registration Period"). The
Company shall be deemed not to have used its reasonable efforts to keep the
Resale Shelf Registration Statement effective during the requisite period if it
voluntarily takes any action that would result in Holders of Securities covered
thereby not being able to offer and sell such Securities during that period,
unless such action is required by applicable law or otherwise permitted
hereunder.

       (c) Notwithstanding any other provisions of this Agreement to the
contrary, the Company shall cause the Resale Shelf Registration Statement and
the related prospectus and any amendment or supplement thereto, as of the
effective date of the Resale Shelf Registration Statement, amendment or
supplement, (i) to comply in all material respects with the applicable
requirements of the Securities Act and the rules and regulations of the
Commission and (ii) not to contain any untrue statement of a material fact or
omit to state a material fact required to be stated therein or necessary in
order to make the statements therein, in light of the circumstances under which
they were made, not misleading.

       2. REGISTRATION PROCEDURES. In connection with the Resale Shelf
Registration contemplated by Section 1 hereof, the following provisions apply:

            (a) The Company shall (i) furnish to the Initial Purchasers, prior
       to the filing thereof with the Commission, a copy of the Resale Shelf
       Registration Statement and each amendment thereof and each supplement, if
       any, to the prospectus included therein and, in the event that the
       Initial Purchasers (with respect to any portion of an unsold allotment
       from the original offering) is participating in the Resale Shelf
       Registration, the Company shall use its reasonable efforts to reflect in
       each such document, when so filed with the Commission, such comments as
       the Initial Purchasers reasonably may propose; and (ii) subject to
       paragraph (l) of this Section 2, include the names of the Holders, who
       propose to sell Securities pursuant to the Resale Shelf Registration
       Statement, as selling securityholders.

            (b) The Company shall give written notice to the Initial Purchasers
       and Holders of the Securities (which notice pursuant to clauses (ii)-(v)
       hereof shall be accompanied by an instruction to suspend the use of the
       prospectus until the requisite changes have been made):

                   (i) when the Resale Shelf Registration Statement or any
            amendment thereto has been filed with the Commission and when the
            Resale Shelf Registration Statement or any post-effective amendment
            thereto has become effective;

                   (ii) of any request by the Commission for amendments or
            supplements to the Resale Shelf Registration Statement or the
            prospectus included therein or for additional information;

                   (iii) of the issuance by the Commission of any stop order
            suspending the effectiveness of the Resale Shelf Registration
            Statement or the initiation of any proceedings for that purpose; and

                   (iv) of the happening of any event that requires the Company
            to make changes in the prospectus which forms a part of the Resale
            Registration Statement in order that the prospectus does not contain
            an untrue statement of a material fact nor omits to state a material
            fact


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            required to be stated therein or necessary to make the statements
            therein, in light of the circumstances under which they were made,
            not misleading.

            (c) The Company shall make every reasonable effort to obtain the
       withdrawal at the earliest possible time, of any order suspending the
       effectiveness of the Resale Shelf Registration Statement.

            (d) The Company shall furnish to each Holder of Securities included
       within the coverage of the Resale Shelf Registration, upon written
       request without charge, at least one copy of the Resale Shelf
       Registration Statement and any post-effective amendment thereto,
       including financial statements and schedules, and, if the Holder so
       requests in writing, all exhibits thereto (including those, if any,
       incorporated by reference).

            (e) The Company shall, during the Resale Shelf Registration Period,
       deliver to each Holder of Securities included within the coverage of the
       Resale Shelf Registration, without charge, as many copies of the
       prospectus included in the Resale Shelf Registration Statement and any
       amendment or supplement thereto as such person may reasonably request.
       The Company consents, subject to the provisions of this Agreement, to the
       use of the prospectus or any amendment or supplement thereto by each of
       the selling Holders of the Securities in connection with the offering and
       sale of the Securities covered by the prospectus, or any amendment or
       supplement thereto, included in the Resale Shelf Registration Statement
       in the manner described therein.

            (f) The Company shall cooperate with the Holders of the Securities
       to facilitate the timely preparation and delivery of certificates
       representing the Securities to be sold pursuant to any Resale Shelf
       Registration Statement (to the extent such Securities are certificated)
       free of any restrictive legends and in such denominations and registered
       in such names as the Holders may reasonably request a reasonable period
       of time prior to sales of the Securities pursuant to such Resale Shelf
       Registration Statement and prior to settlement of such sales.

            (g) Upon the occurrence of any event contemplated by paragraphs (ii)
       through (v) of Section 2(b) above during the Shelf Registration Period,
       the Company shall promptly prepare and file a post-effective amendment to
       the Resale Shelf Registration Statement, a supplement to the related
       prospectus or any other document so that, as thereafter delivered to
       Holders of the Securities or purchasers of Securities, the prospectus
       will not contain an 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 light of the circumstances under which they were
       made, not misleading; PROVIDED HOWEVER, that the Company may delay
       preparing, filing and distributing any such supplement or amendment if
       the Company determines in good faith that such supplement or amendment
       would, in the reasonable judgment of the Company, (i) interfere with or
       affect the negotiation or completion of a transaction that is being
       contemplated by the Company (whether or not a final decision has been
       made to undertake such transaction) or (ii) involve initial or continuing
       disclosure obligations that are not in the best interests of the
       Company's stockholders at such time; PROVIDED FURTHER, that such delay
       shall not extend for a period of more than 30 business days in any three
       month period or more than 60 business days in any twelve month period. If
       the Company notifies the Initial Purchasers and the Holders of the
       Securities in accordance with paragraphs (ii) through (v) of Section 2(b)
       above to suspend the use of the prospectus until the requisite changes to
       the prospectus have been made, then the Initial Purchasers and the
       Holders of the Securities shall suspend use of such prospectus. The
       period of effectiveness of the Resale Shelf Registration Statement
       provided for in Section 1(b) above shall be extended by the number of
       days from and


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       including the date of the giving of such notice to and including the date
       when the Initial Purchasers and the Holders of the Securities shall have
       received such amended or supplemented prospectus pursuant to this Section
       2(g); PROVIDED, HOWEVER, that such period of effectiveness including any
       such extension shall not exceed the holding period applicable to Rule
       144(k) of the Securities Act or any substitution or modification thereof.

            (h) Not later than the effective date of the Resale Shelf
       Registration Statement, the Company will provide CUSIP numbers for the
       Debentures and the Conversion Shares registered under the Resale Shelf
       Registration Statement and provide the Trustee with a certificate for the
       Debentures, in a form eligible for deposit with The Depository Trust
       Company.

            (i) The Company will comply with all rules and regulations of the
       Commission to the extent and so long as they are applicable to the Resale
       Shelf Registration and will make generally available to its security
       holders (or otherwise provide in accordance with Section 11(a) of the
       Securities Act) an earnings statement satisfying the provisions of
       Section 11(a) of the Securities Act, no later than 45 days after the end
       of a 12-month period (or 90 days, if such period is a fiscal year)
       beginning with the first month of the Company's first fiscal quarter
       commencing after the effective date of the Resale Shelf Registration
       Statement, which statement shall cover such 12-month period.

            (j) The Company shall cause the Indenture to be qualified under the
       Trust Indenture Act of 1939, as amended, in a timely manner and
       containing such changes, if any, as shall be necessary for such
       qualification. In the event that such qualification would require the
       appointment of a new trustee under the Indenture, the Company shall
       appoint a new trustee thereunder pursuant to the applicable provisions of
       the Indenture.

            (k) The Company may require each Holder of Securities to be sold
       pursuant to the Resale Shelf Registration Statement to furnish to the
       Company such information regarding the Holder and the distribution of the
       Securities as the Company may from time to time reasonably require for
       inclusion in the Resale Shelf Registration Statement, and the Company may
       exclude from such registration the Securities of any Holder that fails to
       furnish such information within a reasonable time after receiving such
       request.

            (l) The Company shall enter into such customary agreements
       (including, if requested in accordance with Section 7 hereof, an
       underwriting agreement in customary form) and take all such other action,
       if any, as any Holder of the Securities shall reasonably request in order
       to facilitate the disposition of the Securities pursuant to any Resale
       Shelf Registration.

            (m) The Company shall (i) make reasonably available for inspection
       by the Holders, any underwriter participating in any disposition pursuant
       to the Resale Shelf Registration Statement in accordance with Section 7
       hereof and any attorney, accountant or other agent retained by the
       Holders or any such underwriter all relevant financial and other records,
       pertinent corporate documents and properties of the Company and (ii)
       cause the Company's officers, directors, employees, accountants and
       auditors to supply all relevant information reasonably requested by the
       Holders or any such underwriter, attorney, accountant or agent in
       connection with the Resale Shelf Registration Statement, in each case, as
       shall be reasonably necessary to enable such persons, to conduct a
       reasonable investigation within the meaning of Section 11 of the
       Securities Act; provided, however, that the foregoing inspection and
       information gathering shall be coordinated on behalf of the Initial
       Purchasers and the


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       other parties, by one firm of counsel, which firm shall be Cahill Gordon
       & Reindel until another firm shall be designated as described in Section
       3 hereof.

            (n) The Company shall cause (i) its counsel to deliver an opinion
       relating to the Securities in customary form addressed to any
       underwriters dated the closing date of any underwritten offering in
       accordance with Section 7 hereof and covering matters of the type
       customarily in opinions to underwriters in connection with underwritten
       offerings; (ii) its officers to execute and deliver all customary
       documents and certificates and updates thereof requested by any such
       underwriters of the applicable Securities and (iii) its independent
       public accountants and the independent public accountants, if any with
       respect to any other entity for which financial information is provided
       in the Resale Shelf Registration Statement to provide to the selling
       Holders of the applicable Securities and any such underwriter therefor a
       comfort letter in customary form; and covering matters of the type
       customarily covered in comfort letters in connection with underwritten
       offerings, subject to receipt of appropriate documentation as
       contemplated, and only if permitted, by Statement of Auditing Standards
       No. 72.

            (o) In the event that any broker-dealer registered under the
       Exchange Act shall underwrite any Securities or participate as a member
       of an underwriting syndicate or selling group or "assist in the
       distribution" (within the meaning of the Conduct Rules (the "Rules") of
       the National Association of Securities Dealers, Inc. ("NASD")) thereof,
       whether as a Holder of such Securities or as an underwriter, a placement
       or sales agent or a broker or dealer in respect thereof, or otherwise,
       the Company will assist such broker-dealer in complying with the
       requirements of such Rules, including, without limitation, by (i) if such
       Rules, including Rule 2720, shall so require, engaging a "qualified
       independent underwriter" (as defined in Rule 2720) to participate in the
       preparation of the Resale Shelf Registration Statement relating to such
       Securities, to exercise usual standards of due diligence in respect
       thereto and, if any portion of the offering contemplated by such Resale
       Shelf Registration Statement is an underwritten offering in accordance
       with Section 7 hereof or is made through a placement or sales agent, to
       recommend the yield of such Securities, (ii) indemnifying any such
       qualified independent underwriter to the extent of the indemnification of
       underwriters provided in Section 4 hereof and (iii) providing such
       information to such broker-dealer as may be required in order for such
       broker-dealer to comply with the requirements of the Rules.

            (p) The Company shall use its reasonable efforts to take all other
       steps necessary to effect the registration of the Securities covered by
       the Resale Shelf Registration Statement contemplated hereby.

       3. REGISTRATION EXPENSES. The Company shall bear all fees and expenses
incurred in connection with the performance of its obligations under Sections 1
and 2 hereof whether or not a Resale Shelf Registration is filed or becomes
effective including without limitation all registration and filing fees and
expenses and all fees and expenses of compliance with federal securities and
state "blue sky" or securities laws, shall bear or reimburse the Holders of the
Securities covered thereby for the reasonable fees and disbursements of one firm
of counsel, which firm shall be Cahill Gordon & Reindel until another firm shall
be designated by the Holders of a majority in principal amount of the Debentures
covered thereby to act as counsel for the Holders in connection therewith. The
Holders shall be responsible for all other fees and expenses, such as brokerage
fees and commissions.

       4. INDEMNIFICATION. (a) To the extent permitted by law, the Company
agrees to indemnify and hold harmless each Holder of the Securities and each
person, if any, who controls such Holder within the meaning


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of the Securities Act or the Exchange Act (each Holder and such controlling
persons are referred to collectively as the "Indemnified Parties") from and
against any losses, claims, damages or liabilities, joint or several, or any
actions in respect thereof (including, but not limited to, any losses, claims,
damages, liabilities or actions relating to purchases and sales of the
Securities) to which each Indemnified Party may become subject under the
Securities Act, the Exchange Act or otherwise, insofar as such losses, claims,
damages, liabilities or actions arise out of or are based upon any untrue
statement or alleged untrue statement of a material fact contained in the Resale
Shelf Registration Statement or prospectus or in any amendment or supplement
thereto or in any preliminary prospectus relating to the Resale Shelf
Registration, or arise out of, or are based upon, the omission or alleged
omission to state therein a material fact required to be stated therein or
necessary to make the statements therein not misleading, and shall reimburse, as
incurred, the Indemnified Parties for any legal or other expenses reasonably
incurred by them in connection with investigating or defending any such loss,
claim, damage, liability or action in respect thereof; provided, however, that
(i) the Company shall not be liable in any such case to the extent that such
loss, claim, damage or liability arises out of or is based upon any untrue
statement or alleged untrue statement or omission or alleged omission made in
the Resale Shelf Registration Statement or prospectus or in any amendment or
supplement thereto or in any preliminary prospectus relating to the Resale Shelf
Registration in reliance upon and in conformity with written information
pertaining to a Holder and furnished to the Company by or on behalf of such
Holder specifically for inclusion therein, (ii) with respect to any untrue
statement or omission or alleged untrue statement or omission made in any
preliminary prospectus relating to the Resale Shelf Registration Statement, the
indemnity agreement contained in this subsection (a) shall not inure to the
benefit of any Holder from whom the person asserting any such losses, claims,
damages or liabilities purchased the Securities concerned, to the extent that a
prospectus relating to such Securities was required to be delivered by such
Holder under the Securities Act in connection with such purchase and any such
loss, claim, damage or liability of such Holder results from the fact that there
was not sent or given to such person, at or prior to the written confirmation of
the sale of such Securities to such person, a copy of the final prospectus if
the Company had previously furnished copies thereof to such Holder, (iii) the
Company shall not be liable with respect to any statement in, or omission from,
a prospectus used in a manner inconsistent with the second to last sentence of
Section 2(g) and (iv) the Company shall not be liable with respect to any untrue
statement in, or omission from, a prospectus when a subsequent version of the
prospectus had been supplied and the Holders have been given reasonable notice
of such subsequent version prior to the relevant sale or sales; provided,
further, however, that this indemnity agreement will be in addition to any
liability which the Company may otherwise have to such Indemnified Party. The
Company shall also indemnify underwriters, their officers and directors and each
person who controls such underwriters within the meaning of the Securities Act
or the Exchange Act to the same extent as provided above with respect to the
indemnification of the Holders of the Securities if requested by such Holders in
connection with an underwriting in accordance with Section 7 hereof.

       (b) To the extent permitted by law, each Holder of the Securities,
severally and not jointly, will indemnify and hold harmless the Company and each
person, if any, who controls the Company within the meaning of the Securities
Act or the Exchange Act from and against any losses, claims, damages or
liabilities or any actions in respect thereof, to which the Company or any such
controlling person may become subject under the Securities Act, the Exchange Act
or otherwise, insofar as such losses, claims, damages, liabilities or actions
arise out of or are based upon any untrue statement or alleged untrue statement
of a material fact contained in the Resale Shelf Registration Statement or
prospectus or in any amendment or supplement thereto or in any preliminary
prospectus relating to the Resale Shelf Registration, or arise out of or are
based upon the omission or alleged omission to state therein a material fact
necessary to make the statements therein not misleading, but in each case only
to the extent that the untrue statement or omission or alleged untrue statement
or omission was made in reliance upon and in conformity with written information
pertaining to such Holder and furnished


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to the Company by or on behalf of such Holder specifically for inclusion
therein; and, subject to the limitation set forth immediately preceding this
clause, shall reimburse, as incurred, the Company for any legal or other
expenses reasonably incurred by the Company or any such controlling person in
connection with investigating or defending any loss, claim, damage, liability or
action in respect thereof. This indemnity agreement will be in addition to any
liability which such Holder may otherwise have to the Company or any of its
controlling persons.

       (c) Promptly after receipt by an indemnified party under this Section 4
of notice of the commencement of any action or proceeding (including a
governmental investigation), such indemnified party will, if a claim in respect
thereof is to be made against the indemnifying party under this Section 4,
notify the indemnifying party of the commencement thereof; but the omission so
to notify the indemnifying party will not, in any event, relieve the
indemnifying party from any obligations to any indemnified party other than the
indemnification obligation provided in paragraph (a) or (b) above. In case any
such action is brought against any indemnified party, and it notifies the
indemnifying party of the commencement thereof, the indemnifying party will be
entitled to participate therein and, to the extent that it may wish, jointly
with any other indemnifying party similarly notified, to assume the defense
thereof, with counsel reasonably satisfactory to such indemnified party (who
shall not, except with the consent of the indemnified party, be counsel to the
indemnifying party), and after notice from the indemnifying party to such
indemnified party of its election so to assume the defense thereof the
indemnifying party will not be liable to such indemnified party under this
Section 4 for any legal or other expenses, other than reasonable costs of
investigation, subsequently incurred by such indemnified party in connection
with the defense thereof. No indemnifying party shall, without the prior written
consent of the indemnified party, effect any settlement of any pending or
threatened action in respect of which any indemnified party is or could have
been a party and indemnity could have been sought hereunder by such indemnified
party unless such settlement includes an unconditional release of such
indemnified party from all liability on any claims that are the subject matter
of such action. An indemnifying party will not be liable for any settlement of
any action or claim effected without its written consent; provided, however,
that such consent will not be reasonably withheld.

       (d) If the indemnification provided for in this Section 4 is unavailable
or insufficient to hold harmless an indemnified party under subsections (a) or
(b) above, then each indemnifying party shall contribute to the amount paid or
payable by such indemnified party as a result of the losses, claims, damages or
liabilities (or actions in respect thereof) referred to in subsection (a) or (b)
above (i) in such proportion as is appropriate to reflect the relative benefits
received by the indemnifying party or parties on the one hand and the
indemnified party on the other from the sale of the Securities, pursuant to the
Resale Shelf Registration, or (ii) if the allocation provided by the foregoing
clause (i) 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 party or parties on the
one hand and the indemnified party on the other in connection with the
statements or omissions that resulted in such losses, claims, damages or
liabilities (or actions in respect thereof) as well as any other relevant
equitable considerations. 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 on the one hand or
such Holder or such other indemnified party, as the case may be, on the other,
and the parties' relative intent, knowledge, access to information and
opportunity to correct or prevent such statement or omission. The amount paid by
an indemnified party as a result of the losses, claims, damages or liabilities
referred to in the first sentence of this subsection (d) shall be deemed to
include any legal or other expenses reasonably incurred by such indemnified
party in connection with investigating or defending any action or claim which is
the subject of this subsection (d). Notwithstanding any other provision of this
Section 4(d), the Holders of the Securities


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shall not be required to contribute any amount in excess of the amount by which
the net proceeds received by such Holders from the sale of the Securities
pursuant to a Resale Shelf Registration Statement exceeds the amount of damages
which such Holders 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. For purposes of this paragraph (d),
each person, if any, who controls such indemnified party within the meaning of
the Securities Act or the Exchange Act shall have the same rights to
contribution as such indemnified party and each person, if any, who controls the
Company within the meaning of the Securities Act or the Exchange Act shall have
the same rights to contribution as the Company.

       (e) The agreements contained in this Section 5 shall survive the sale of
the Securities pursuant to the Resale Shelf Registration Statement and shall
remain in full force and effect, regardless of any termination or cancellation
of this Agreement or any investigation made by or on behalf of any indemnified
party.

       5. ADDITIONAL INTEREST UNDER CERTAIN CIRCUMSTANCES. (a) Additional
interest (the "Additional Interest") with respect to the Debentures shall be
assessed as follows if any of the following events occur (each such event in
clauses (i) through (iii) below a "Registration Default"):

            (i) If on or prior to the 90th day after the first date of original
       issuance of the Debentures, the Resale Shelf Registration Statement has
       not been filed with the Commission;

            (ii) If on or prior to the 180th day after the first date of
       original issuance of the Debentures, the Resale Shelf Registration
       Statement has not been declared effective by the Commission; or

            (iii) If after the Resale Shelf Registration Statement is declared
       effective (A) such Resale Shelf Registration Statement thereafter ceases
       to be effective; or (B) such Resale Shelf Registration Statement or the
       related prospectus ceases to be usable (except as permitted in paragraph
       (b)) in connection with resales of Transfer Restricted Securities during
       the periods specified herein because either (1) any event occurs as a
       result of which the related prospectus forming part of such Resale Shelf
       Registration Statement would include any untrue statement of a material
       fact or omit to state any material fact necessary to make the statements
       therein in the light of the circumstances under which they were made not
       misleading, or (2) it shall be necessary to amend such Resale Shelf
       Registration Statement or supplement the related prospectus, to comply
       with the Securities Act or the Exchange Act or the respective rules
       thereunder.

Each of the foregoing will constitute a Registration Default whatever the reason
for any such event and whether it is voluntary or involuntary or is beyond the
control of the Company or pursuant to operation of law or as a result of any
action or inaction by the Commission.

Additional Interest shall accrue on the Debentures over and above the interest
set forth in the title of the Securities that are, in each case, Transfer
Restricted Securities, from and including the date on which any such
Registration Default shall occur to but excluding the date on which all such
Registration Defaults have been cured, at a rate of 0.25% per annum in the case
of (i) and 0.50% per annum in the case of (ii) or (iii).

       (b) A Registration Default referred to in Section 5(a)(iii)(B) hereof
shall be deemed not to have occurred and be continuing in relation to the Resale
Shelf Registration Statement or the related prospectus if


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(i) such Registration Default has occurred solely as a result of (x) the filing
of a post-effective amendment to such Resale Shelf Registration Statement to
incorporate annual audited financial information with respect to the Company
where such post-effective amendment is not yet effective and needs to be
declared effective to permit Holders to use the related prospectus or (y) other
material events, with respect to the Company that would need to be described in
such Resale Shelf Registration Statement or the related prospectus and (ii) in
the case of clause (y), the Company is proceeding in good faith in accordance
with Section 2(h); provided, however, that in any case if such Registration
Default occurs for a continuous period in excess of 30 days, Additional Interest
shall be payable in accordance with the above paragraph from the day such
Registration Default occurs until such Registration Default is cured.

       (c) Any amounts of Additional Interest due pursuant to clause (i), (ii)
or (iii) of Section 5(a) above will be payable in cash on the regular interest
payment dates with respect to the Debentures. The amount of Additional Interest
will be determined by multiplying the applicable Additional Interest rate by the
principal amount of the Debentures, multiplied by a fraction, the numerator of
which is the number of days such Additional Interest rate was applicable during
such period (determined on the basis of a 360-day year comprised of twelve
30-day months), and the denominator of which is 360.

       (d) "Transfer Restricted Securities" means each Security until (i) the
date on which such Security has been effectively registered under the Securities
Act and disposed of in accordance with the Resale Shelf Registration Statement
or (ii) the date on which such Security is distributed to the public pursuant to
Rule 144 under the Securities Act or is saleable pursuant to Rule 144(k) under
the Securities Act.

       (e) Except as set forth in this Section 5, the Company will have no other
liabilities for monetary damages with respect to its registration obligations
hereunder; PROVIDED, HOWEVER, that in the event the Company breaches, fails to
comply with or violates the provisions of this Agreement, the Holders shall be
entitled to, and the Company shall not oppose the granting of, equitable relief,
including injunction and specific performance.

       6. RULES 144 AND 144A. The Company shall use its reasonable efforts to
file the reports required to be filed by it under the Securities Act and the
Exchange Act in a timely manner and, if at any time the Company is not required
to file such reports, it will, upon the request of any Holder of Debentures,
make publicly available such other information as is necessary to permit sales
of their securities pursuant to Rules 144 and 144A for the Shelf Registration
Period. The Company covenants that it will take such further action as any
Holder of Debentures may reasonably request, all to the extent required from
time to time to enable such Holder to sell Debentures without registration under
the Securities Act within the limitation of the exemptions provided by Rules 144
and 144A (including the requirements of Rule 144A(d)(4)). The Company will
provide a copy of this Agreement to prospective purchasers of Debentures
identified to the Company by the Initial Purchasers upon request. Upon the
request of any Holder of Debentures, the Company shall deliver to such Holder a
written statement as to whether it has complied with such requirements.
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.

       7. UNDERWRITTEN REGISTRATIONS. The Holders of Securities covered by the
Resale Shelf Registration Statement who desire to do so may sell such securities
in an underwritten offering in accordance with the conditions set forth below.
In any such underwritten offering, the investment banker or bankers and manager
or managers that will administer the offering will be selected by, and the
underwriting arrangements with respect thereto will be approved by, the Holders
of a majority of the Securities to be included in such offering; provided,
however, that (i) with respect to the investment bankers and managers, such
investment bankers and

                                       9

<PAGE>


managers will be selected by such Holders subject to approval by the Company,
which approval shall not be unreasonably withheld, and (ii) the Company shall
not be obligated to arrange for more than one underwritten offering during the
Shelf Registration Period. No Holder may participate in any underwritten
offering contemplated hereby unless such Holder (a) agrees to sell such Holder's
Securities in accordance with any approved underwriting arrangements, (b)
completes and executes all reasonable questionnaires, powers of attorney,
indemnities, underwriting agreements, lock-up letters and other documents
required under the terms of such approved underwriting arrangements, and (c) at
least 25% of the outstanding Transfer Restricted Securities are included in such
underwritten offering. The Holders participating in any underwritten offering
shall be responsible for any expenses customarily borne by selling
securityholders, including underwriting discounts and commissions and fees and
expenses of counsel to the selling securityholders to the extent not required to
be paid by the Company pursuant to Section 3 hereof. Notwithstanding the
foregoing or the provisions of Section 2(l) hereof, upon receipt of a request
from the managing underwriter or a representative of Holders of a majority of
the Transfer Restricted Securities outstanding to prepare and file an amendment
or supplement to the Resale Shelf Registration Statement in connection with an
underwritten offering, the Company may delay the filing of any such amendment or
supplement as set forth in Section 2(h).

       8. MISCELLANEOUS.

       (a) 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, except by the Company and the written
consent of the Holders of a majority in principal amount of the Securities
(provided that Holders of Common Stock issued upon conversion of Debentures
shall not be deemed Holders of Common Stock, but shall be deemed to be Holders
of the aggregate principal amount at maturity of Debentures from which such
Common Stock was converted) affected by such amendment, modification,
supplement, waiver or consents. Without the consent of the Holder of each
Debenture, however, no modification may change the provisions relating to the
payment of Additional Interest.

       (b) NOTICES. All notices and other communications provided for or
permitted hereunder shall be made in writing by hand delivery, first-class mail,
facsimile transmission, or air courier which guarantees overnight delivery:

            (1) if to a Holder of the Securities, at the most current address
       given by such Holder to the Company.

            (2) if to the Initial Purchasers:

                           Credit Suisse First Boston Corporation
                           Eleven Madison Avenue
                           New York, NY 10010-3629
                           Fax No.:  (212) 892-0776
                           Attention:  Transactions Advisory Group


                                       10

<PAGE>


            with a copy to:

                           Cahill Gordon & Reindel
                           80 Pine Street
                           New York, NY  10005
                           Fax No.:  (212) 269-5420
                           Attention:  Geoffrey E. Liebmann

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

                           Genzyme Corporation
                           One Kendall Square
                           Cambridge, MA  02139
                           Fax No.:  (617) 252-7553
                           Attention:  Chief Financial Officer

            with a copy to:

                           Palmer & Dodge LLP
                           One Beacon Street
                           Boston, MA  02108
                           Fax No.:  (617) 227-4420
                           Attention:  Paul Kinsella

       All such notices and communications shall be deemed to have been duly
given: at the time delivered by hand, if personally delivered; three business
days after being deposited in the mail, postage prepaid, if mailed; when receipt
is acknowledged by recipient's facsimile machine operator, if sent by facsimile
transmission; and on the day delivered, if sent by overnight air courier
guaranteeing next day delivery.

       (c) NO INCONSISTENT AGREEMENTS. The Company will not on or after the date
of this Agreement enter into any agreement with respect to its securities that
is inconsistent with the rights granted to the Holders in this Agreement or
otherwise conflicts with the provisions hereof. The rights granted to the
Holders hereunder do not in any way conflict with and are not inconsistent with
the rights granted to the holders of the Company's securities under any
agreement in effect on the date hereof.

       (d) SUCCESSORS AND ASSIGNS. This Agreement shall be binding upon the
Company and its successors and assigns.

       (e) 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.

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


                                       11

<PAGE>


       (g) GOVERNING Law. THIS AGREEMENT SHALL BE GOVERNED BY, AND CONSTRUED IN
ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK WITHOUT REGARD TO PRINCIPLES
OF CONFLICTS OF LAWS.

       (h) SEVERABILITY. If any one or more of the provisions contained herein,
or the application thereof in any circumstance, is held invalid, illegal or
unenforceable, the validity, legality and enforceability of any such provision
in every other respect and of the remaining provisions contained herein shall
not be affected or impaired thereby.

       (i) SECURITIES HELD BY THE COMPANY. Whenever the consent or approval of
Holders of a specified percentage of principal amount of Securities is required
hereunder, Securities held by the Company or its affiliates (other than
subsequent Holders of Securities if such subsequent Holders are deemed to be
affiliates solely by reason of their holdings of such Securities) shall not be
counted in determining whether such consent or approval was given by the Holders
of such required percentage.

       (j) THIRD PARTY BENEFICIARIES. The Holders shall be third party
beneficiaries to the agreements made hereunder between the Company, on the one
hand, and the Initial Purchasers, on the other hand, and shall have the right to
enforce such agreements directly to the extent they may deem such enforcement
necessary or advisable to protect their rights or the rights of Holders
hereunder.


                                       12

<PAGE>


       If the foregoing is in accordance with your understanding of our
agreement, please sign and return to the Company a counterpart hereof, whereupon
this instrument, along with all counterparts, will become a binding agreement on
the Initial Purchasers and the Company in accordance with its terms.

                                       Very truly yours,

                                       GENZYME CORPORATION


                                       By:
                                          -------------------------------------
                                          Name:
                                          Title:


The foregoing Registration Rights
Agreement is hereby confirmed and
accepted as of the date first above
written.

CREDIT SUISSE FIRST BOSTON CORPORATION
GOLDMAN, SACHS & CO.
SALOMON SMITH BARNEY INC.


         By: CREDIT SUISSE FIRST BOSTON CORPORATION


         By:
            ---------------------------------------
            Name:
            Title: