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                                                                 Exhibit 10.28.6

                       THIRD AMENDMENT TO CREDIT AGREEMENT
              AND AMENDMENT TO REVOLVING CREDIT NOTE AND TERM NOTE

     Reference is hereby made to that certain Credit Agreement dated as of
December 28, 1998, as amended, by and between Genzyme Transgenics Corporation
(the "Borrower"), and Fleet National Bank ("Lender") (as may be further amended
from time to time, the "Credit Agreement"), together with that certain Revolving
Credit Note and Term Note of even date therewith. Capitalized terms not defined
herein shall have the meanings ascribed thereto in the Credit Agreement.

     WHEREAS, the Borrower and Guarantor have requested that the Lender agree to
modify the definition of Termination Date to extend the maturity of the Loans
evidenced by the Credit Agreement and other Loan Documents from January 11, 2002
to March 28, 2002, and the Lender has agreed to such change, subject to the
terms and conditions of this Third Amendment To Credit Agreement And Amendment
To Revolving Credit Note And Term Note (the "Third Amendment").

     NOW, THEREFORE, for good and valuable consideration, the receipt of which
is hereby acknowledged, the parties do hereby agree as follows:

1.   AMENDMENTS.

     1.1. Schedule 1 to the Credit Agreement is hereby amended to delete the
defined term "Termination Date" and to replace such defined term with the
following:

          "`TERMINATION DATE'" - the earlier of (a) March 28, 2002, and (b) the
          date the Lender's commitment to make Loans is terminated pursuant to
          Section 8.2 of Article 8."

     1.2. The "Maturity Date", as defined in the Revolving Credit Note and the
Term Note, is extended to March 28, 2002.

     1.3. Section 4.2 of the Term Note shall be deleted and replaced with the
following:

          "4.2 PRINCIPAL. The principal amount of the Term Loan outstanding on
               January 11, 2002 (which principal amount equals $5,734,795.20)
               shall be repaid in two installments, as follows: (a) on January
               31, 2002, in the amount of $177,500 of and (b) on March 28, 2002,
               in the amount of the then outstanding principal."

2.   REPRESENTATIONS AND WARRANTIES.  In order to induce the Lender to enter
into this Third Amendment, the Borrower makes the following representations and
warranties, all of which shall survive the execution and delivery of this Third
Amendment:

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     (a)  The Borrower has all requisite corporate, partnership or other power
and authority to execute, deliver and perform its obligations under this Third
Amendment and under the Notes and the Credit Agreement, as amended hereby. This
Third Amendment has been duly authorized, executed and delivered by the
Borrower, and does not conflict with, violate or result in a breach of or
require any consent under any applicable law, rule or regulation or any of the
terms of the charter or by-laws (or equivalent constitutional documents) of the
Borrower, any agreement or instrument to which the Borrower or any of its
Subsidiaries is a party or to which any of them or their property is bound or to
which any of them is subject. This Third Amendment, the Notes, and the Credit
Agreement, as amended hereby, constitute the legal, valid and binding obligation
of the Borrower enforceable against it in accordance with its terms.

     (b)  On the date hereof each of the representations and warranties in the
Credit Agreement are true, accurate and complete in all material respects except
as to changes that have been consented to or approved by Lender in writing,
PROVIDED THAT, if any representation or warranty is expressly required in the
Credit Agreement to be made only as of a specific date, such representation or
warranty shall be true, accurate and complete as of such date in all material
respects.

     (c)  Upon the execution and delivery of this Third Amendment, and the
satisfaction of each of the conditions precedent set forth in Section 4 of this
Third 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 on the date (the "Effective Date")
when the Borrower and the Lender shall have executed this Third Amendment and
when each of the following conditions shall have been fulfilled:

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

     (b)  PROCEEDINGS; RECEIPT OF DOCUMENTS. All requisite corporate action and
proceedings of the Borrower in connection with the execution and delivery of
this Third Amendment and the other Third Amendment Documents shall be
satisfactory in form and substance to the Lender and its counsel, and the Lender
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 Lender or its counsel may have requested in connection
therewith, such documents where requested by the Lender 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 Borrower, threatened litigation with respect to the Borrower or
the Guarantor before any court,

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arbitrator or governmental or administrative body or agency which challenges or
relates to (i) the transactions contemplated hereby or (ii) the Loan Documents.

     (d)  GUARANTOR CONSENT. The Lender shall have received a copy of a letter
from the Guarantor in which the Guarantor shall have consented to this Third
Amendment, which letter shall be in form and substance satisfactory to Lender.

4.   REAFFIRMATION AND RATIFICATION OF EXISTING AGREEMENTS, ETC.  The Borrower:
(i) reaffirms and ratifies all the Obligations to the Lender, in respect of the
Credit Agreement and Notes, 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
and Notes, as amended hereby, and the other Loan Documents shall remain in full
force and effect, enforceable against the Borrower in accordance with its terms.

5.   MISCELLANEOUS.

     (a)  This Third 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 Third 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 Third Amendment are
inserted for convenience only and shall not in any way affect the meaning or
construction of any provision of this Third Amendment.

     (d)  This Third Amendment, together with the other Third Amendment
Documents, 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 Third Amendment, together with the other Third Amendment
Documents, shall be deemed to be Loan Documents under the Credit Agreement.

     (f)  EACH OF THE BORROWER AND THE LENDER 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 THIRD AMENDMENT, OR
THE TRANSACTIONS CONTEMPLATED HEREBY.

     (g)  The Borrower 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 Lender in connection with the negotiation,
documentation, administration and enforcement of this Third Amendment.

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     IN WITNESS WHEREOF, this Third Amendment has been duly executed and
delivered as a sealed instrument at Boston, Massachusetts as of this 11th day of
January, 2002.

                                                THE BORROWER:

                                                GENZYME TRANSGENICS CORPORATION


                                                By: /s/ John B. Green
                                                    ---------------------------
                                                    Title: Vice President

                                                THE LENDER:

                                                FLEET NATIONAL BANK


                                                By: /s/ Kimberly Martone
                                                    ---------------------------
                                                    Title: Managing Director