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                                                                EXHIBIT 10.27.5


                             FOURTH AMENDMENT TO
                          REVOLVING CREDIT AGREEMENT


     THIS FOURTH AMENDMENT TO REVOLVING CREDIT AGREEMENT (this "Fourth
Amendment") is made and entered into as of the 1st day of October, 1996, by and
among GENZYME TRANSGENICS CORPORATION, a Massachusetts corporation having its
principal place of business at One Mountain Road, Framingham, Massachusetts 
01701 (the "Parent") and its Subsidiaries listed on the signature pages hereto
(the Parent and each such Subsidiary is individually referred to herein as a
"Borrower," and collectively as the "Borrowers"), and THE FIRST NATIONAL BANK OF
BOSTON (the "Bank"), a national banking association with its head office at 100
Federal Street, Boston, Massachusetts 02110.

     WHEREAS, the Borrowers and the Bank entered into a Revolving Credit
Agreement dated as of July 3, 1995 as amended by the First Amendment to
Revolving Credit Agreement dated as of September 15, 1995, the Second Amendment
to Revolving Credit Agreement dated as of December 22, 1995, and the Third
Amendment to Revolving Credit Agreement dated as of March 29, 1996 (as further
amended and in effect from time to time, the "Credit Agreement") pursuant to
which the Bank extended credit to the Borrowers on the terms set forth therein;

     WHEREAS, the Bank and the Borrowers have agreed to amend the Credit
Agreement as hereinafter set forth;

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

     1.   Definitions.  Capitalized terms used herein without definition
have the meanings ascribed to them in the Credit Agreement.

     2.   Amendment to Credit Agreement.

     The following new subsection (ix) is added to Section 8.2(a) of the
Credit Agreement:

     "and (ix) Indebtedness of TSI Corporation to Financing for Science
     International, Inc. pursuant to the terms of the Master Equipment
     Lease Agreement dated as of September 27, 1994 by and between TSI
     Corporation and Financing for Science International, Inc.; provided
     that the aggregate outstanding amount of all such Indebtedness
     shall not exceed $5,200,000."

     3.   Ratification, etc. 

     Except as expressly amended hereby, the Credit Agreement, the other
Loan Documents and all documents, instruments and agreements related thereto are
hereby ratified and confirmed in all respects and shall continue in full force
and effect.  This Fourth Amendment and the Credit Agreement shall hereafter be
read and construed together as a 


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single document, and all references in the Credit Agreement or any related
agreement or instrument to the Credit Agreement shall hereafter refer to the
Credit Agreement as amended by this Fourth Amendment.  By executing this Fourth
Amendment where indicated below, the Guarantor hereby ratifies and confirms its
guaranty of the Obligations pursuant to the terms of the Guaranty, as amended,
and acknowledges and consents to the terms of this Fourth Amendment.

     4.   GOVERNING LAW.

     THIS FOURTH AMENDMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE COMMONWEALTH OF MASSACHUSETTS AND SHALL TAKE
EFFECT AS A SEALED INSTRUMENT IN ACCORDANCE WITH SUCH LAWS.

     5.   Counterparts.  This Fourth Amendment may be executed in any
number of counterparts and by different parties hereto on separate counterparts,
each of which when so executed and delivered shall be an original, but all of
which counterparts taken together shall be deemed to constitute one and the same
instrument.  A complete set of counterparts shall be lodged with the Bank.

     6.   Effectiveness.  This Fourth Amendment shall become effective
upon its execution and delivery by the respective parties hereto.

     7.   Entire Agreement.  THE CREDIT AGREEMENT AS AMENDED REPRESENTS
THE FINAL AGREEMENT BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE
OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES.  THERE
ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.


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     IN WITNESS WHEREOF, the undersigned have duly executed this
Agreement under seal as of October 1, 1996.

                                  THE BORROWERS:

                                  GENZYME TRANSGENICS CORPORATION



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


                                  Address of the Borrower:

                                  One Mountain Road
                                  Framingham, MA  01701
                                  Tel:  (508) 872-8400
                                  Fax:  (508) 872-0827


                                  TSI CORPORATION


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


                                  TSI MASON LABORATORIES INC.


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


                                  TSI WASHINGTON LABORATORIES, INC.



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


                                        - 3 -
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                                  TSI REDFIELD LABORATORIES, INC.




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


                                  ARGUS RESEARCH LABORATORIES, INC.



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


                                  TRANSGENIC INVESTMENTS, INC.



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


                                  HEALTH AND SCIENCES RESEARCH
                                   INCORPORATED



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


                                  THE TSI CENTER FOR DIAGNOSTIC
                                   PRODUCTS, INC.




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


                                        - 4 -


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                                  BIODEVELOPMENT LABORATORIES, INC.



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


                                  THE BANK:

                                  THE FIRST NATIONAL BANK OF BOSTON



                                  By: /s/ Elizabeth C. Everett
                                      ----------------------------
                                      Name:  Elizabeth C. Everett
                                      Title:  Vice President

                                  Address:

                                  100 Federal Street
                                  Boston, MA 02110
                                  Tel:   617-434-2318
                                  Fax:  617-434-0819  



ACCEPTED AND AGREED
TO BY:

The Guarantor:

GENZYME CORPORATION


By: /s/ Evan M. Lebson
    ------------------
Title:  Vice President and Treasurer


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