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                                                                  Exhibit 10.1




                       FIRST AMENDMENT TO CREDIT AGREEMENT

         FIRST AMENDMENT TO CREDIT AGREEMENT ("First Amendment") dated as of
June 30, 2004 among and between Genzyme Corporation (the "Company"); each of the
financial institutions identified under the caption "Lenders" on the signature
pages hereto; and Fleet National Bank ("Fleet") as administrative agent for the
Lenders (the "Administrative Agent").

         Reference is made to the Credit Agreement dated as of December 10,
2003, among and between the Company, the Subsidiary Guarantors party thereto,
the Lenders, and the Administrative Agent, pursuant to which the Lenders
furnished to the Company a $350,000,000 revolving line of credit (as amended
hereby, the "Credit Agreement"). Capitalized terms used in this First Amendment
have the meanings given such terms in the Credit Agreement, as amended hereby,
except as provided otherwise herein.

         The Company has requested that the Credit Agreement be amended in
respect of the amount of additional Indebtedness the Company and its
Subsidiaries may incur, as set forth below. Under Section 12.5 of the Credit
Agreement, this First Amendment, to be effective, must be signed by the Company,
the Administrative Agent and the Lenders constituting the Required Lenders.

1. AMENDMENT: SECTION 9.7(J)--ADDITIONAL INDEBTEDNESS. Section 9.7(j) of the
Credit Agreement is hereby amended to replace the figure "$700,000,000", both
times it appears in such section, with the figure "$800,000,000" in lieu
thereof.

2. REPRESENTATIONS AND WARRANTIES. In order to induce the Administrative Agent
and Required Lenders to enter into this First Amendment, the Company makes the
following representations and warranties, all of which shall survive the
execution and delivery of this First Amendment:

         (a) The Company has all requisite corporate power and authority to
execute, deliver and perform its obligations under this First Amendment and
under the Credit Agreement, as amended hereby. This First Amendment has been
duly authorized, executed and delivered by the Company, and does not conflict
with, violate or result in a breach of or require any consent under (i) any
applicable law or regulation or any of the terms of the charter or by-laws of
the Company, or (ii) any agreement or instrument to which the Company or any
Subsidiary is a party or to which any of them or their Property is bound or to
which any of them is subject; except to the extent, with respect to the
foregoing clause (ii), any such conflict, violation, or breach, or the failure
to have any such consent, (x) could not reasonably be expected (either
individually or in the aggregate) to have a Material Adverse Effect and (y) does
not and will not result in any liability of the Administrative Agent or any
Lender or in the acceleration or required prepayment of any Indebtedness or the
termination of any commitments to extend credit. This First Amendment and the
Credit Agreement, as amended hereby, constitute the legal, valid and binding
obligation of the Company enforceable against the Company in accordance with its
terms.


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         (b) On the date hereof each of the representations and warranties in
the Credit Agreement are true, accurate and complete in all material respects
(other than those representations and warranties made as of a specific date,
which were true, accurate and complete in all material respects as of such
specific date).

         (c) Upon the execution and delivery of this First Amendment, and the
satisfaction of each of the conditions precedent set forth in Section 3 of this
First Amendment, no Default or Event of Default shall exist and be continuing.

3. CONDITIONS PRECEDENT. The agreements contained herein and the amendments
contemplated hereby shall become effective when the Company, the Required
Lenders, and the Administrative Agent shall have executed this First Amendment
and when each of the following conditions shall have been fulfilled (the
"Effective Date"):

         (a) EXECUTION OF DOCUMENTS, ETC. This First Amendment and any other
agreements, documents and instruments to be executed and/or delivered in
connection herewith (collectively the "First Amendment Documents") shall have
been duly and properly authorized and executed by: the Company, the
Administrative Agent, and the Required Lenders and shall be in full force and
effect on and as of the Effective Date of this First Amendment and all
representations and warranties of the Company hereunder shall continue to be
true, accurate and complete.

         (b) PROCEEDINGS; RECEIPT OF DOCUMENTS. All requisite corporate action
and proceedings of the Company in connection with the execution and delivery of
this First Amendment shall be satisfactory in form and substance to the
Administrative Agent and its counsel, and the Administrative Agent and its
counsel shall have received all information and copies of all documents,
including without limitation, records of requisite corporate action and
proceedings which the Administrative Agent or its counsel may have requested in
connection therewith, such documents where requested by the Administrative Agent
or its counsel to be certified by appropriate persons or governmental
authorities.

         (c) MATERIAL LITIGATION. There shall be no pending or, to the best
knowledge of the Company, threatened litigation with respect to the Company
before any court, arbitrator or governmental or administrative body or agency
which challenges or relates to (i) the lending transactions contemplated hereby
or (ii) the Loan Documents.

         (d) FEES. The Company shall have reimbursed the Administrative Agent on
account of the reasonable fees and expenses of counsel to the Administrative
Agent.

4. REAFFIRMATION AND RATIFICATION OF EXISTING AGREEMENTS, ETC. The Company: (i)
reaffirms and ratifies all the Obligations to the Agents and the Lenders, in
respect of the Credit Agreement, as hereby amended, and the other Loan
Documents, (ii) certifies that there are no defenses, offsets or counterclaims
to such Obligations as of the date hereof, (iii) expressly acknowledges its
continuing liability pursuant thereto, and (iv) agrees that each of the Credit
Agreement, as amended hereby, and the other Loan Documents shall remain in full
force and effect, enforceable against the Company in accordance with its terms.


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5. MISCELLANEOUS.

         (a) This First Amendment may be executed on separate counterparts by
the parties hereto, each of which when so executed and delivered shall be an
original, but all of which shall constitute one and the same agreement.

         (b) This First Amendment and the rights and obligations of the parties
hereunder shall be construed in accordance with and be governed by the laws of
the Commonwealth of Massachusetts (without giving effect to the conflict of law
principles thereof).

         (c) The headings of the several sections of this First Amendment are
inserted for convenience only and shall not in any way affect the meaning or
construction of any provision of this First Amendment.

         (d) This First Amendment embodies the entire agreement and
understanding among the parties relating to the subject matter hereof and
supersedes all prior proposals, negotiation, agreements and understandings
relating to such subject matter.

         (e) This First Amendment shall be deemed to be a Loan Document under
the Credit Agreement.

         (f) EACH OF THE OBLIGORS, THE AGENTS AND THE LENDERS HEREBY IRREVOCABLY
WAIVES, TO THE FULLEST EXTENT PERMITTED BY APPLICABLE LAW, ANY AND ALL RIGHT TO
TRIAL BY JURY IN ANY LEGAL PROCEEDING ARISING OUT OF OR RELATING TO THIS FIRST
AMENDMENT, OR THE TRANSACTIONS CONTEMPLATED HEREBY.

         (g) The Company shall pay on demand the reasonable costs and expenses,
including, without limitation, reasonable attorneys' fees and expenses incurred,
or which may be incurred by the Agents or the Lenders in connection with the
negotiation, documentation, administration and enforcement of this First
Amendment.



            [THE REMAINDER OF THIS PAGE IS INTENTIONALLY LEFT BLANK]


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         IN WITNESS WHEREOF, this First Amendment has been duly executed and
delivered as a sealed instrument at Boston, Massachusetts as of the date first
above written.

                                     THE COMPANY:

                                     GENZYME CORPORATION

                                     By: /s/ MICHAEL S. WYZGA
                                         --------------------------------------
                                         Title:  Executive Vice President and
                                                 Chief Financial Officer


                                     THE SUBSIDIARY GUARANTOR:

                                     SANGSTAT MEDICAL CORPORATION

                                     By: /s/ GEORGES GEMAYEL
                                         --------------------------------------
                                         Title:  President


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                                     ADMINISTRATIVE AGENT:

                                     FLEET NATIONAL BANK

                                     By: /s/ ANTHEA DEL BIANCO
                                         --------------------------------------
                                         Title:  Vice President


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LENDERS:

         The undersigned Lender, with the Revolving Credit Commitment set forth
below, hereby enters into the foregoing First Amendment To Credit Agreement
dated as of June 30, 2004.


Revolving Credit Commitment:                FLEET NATIONAL BANK


$50,000,000.00                              By: /s/ CRAIG MURLLESS
--------------                                  -------------------------------
                                                Title:  Principal








             [Signature Page to First Amendment to Credit Agreement]


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         The undersigned Lender, with the Revolving Credit Commitment set forth
below, hereby enters into the foregoing First Amendment To Credit Agreement
dated as of June 30, 2004.



Revolving Credit Commitment:                THE BANK OF NOVA SCOTIA


$35,000,000.00                              By: /s/ CAROLYN A. CALLOWAY
--------------                                  -------------------------------
                                                Title:  Managing Director








             [Signature Page to First Amendment to Credit Agreement]


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         The undersigned Lender, with the Revolving Credit Commitment set forth
below, hereby enters into the foregoing First Amendment To Credit Agreement
dated as of June 30, 2004.



Revolving Credit Commitment:                CITIZENS BANK OF MASSACHUSETTS


$35,000,000.00                              By: /s/ R. SCOTT HASKELL
--------------                                  -------------------------------
                                                Title:  Vice President






             [Signature Page to First Amendment to Credit Agreement]


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         The undersigned Lender, with the Revolving Credit Commitment set forth
below, hereby enters into the foregoing First Amendment To Credit Agreement
dated as of June 30, 2004.



Revolving Credit Commitment:                WACHOVIA BANK, National Association


$35,000,000.00                              By: /s/ JEANETTE A. GRIFFIN
--------------                                  -------------------------------
                                                Title:  Director








             [Signature Page to First Amendment to Credit Agreement]


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         The undersigned Lender, with the Revolving Credit Commitment set forth
below, hereby enters into the foregoing First Amendment To Credit Agreement
dated as of June 30, 2004.



Revolving Credit Commitment:                BANK OF TOKYO-MITSUBISHI TRUST
                                            COMPANY


$30,000,000.00                              By: /s/ LILLIAN KIM
--------------                                  -------------------------------
                                                Title:  Vice President







             [Signature Page to First Amendment to Credit Agreement]


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         The undersigned Lender, with the Revolving Credit Commitment set forth
below, hereby enters into the foregoing First Amendment To Credit Agreement
dated as of June 30, 2004.



Revolving Credit Commitment:                KEYBANK NATIONAL ASSOCIATION



$25,000,000.00                              By: /s/  JAMES A. TAYLOR
--------------                                  -------------------------------
                                                Title:  Vice President







             [Signature Page to First Amendment to Credit Agreement]


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         The undersigned Lender, with the Revolving Credit Commitment set forth
below, hereby enters into the foregoing First Amendment To Credit Agreement
dated as of June 30, 2004.



Revolving Credit Commitment:                MELLON BANK, N.A.


$25,000,000.00                              By: /s/  J. WADE BELL
--------------                                  -------------------------------
                                                Title:  Vice President








             [Signature Page to First Amendment to Credit Agreement]


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         The undersigned Lender, with the Revolving Credit Commitment set forth
below, hereby enters into the foregoing First Amendment To Credit Agreement
dated as of June 30, 2004.



Revolving Credit Commitment:                THE GOVERNOR AND COMPANY OF THE
                                            BANK OF IRELAND


$15,000,000.00                              By: /s/  EOGHAM DOYLE
--------------                                  -------------------------------
                                                Title:  Deputy Manager







             [Signature Page to First Amendment to Credit Agreement]