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                                                                 EXHIBIT 10.27.6


                                                                               
                                  FIFTH AMENDMENT TO
                              REVOLVING CREDIT AGREEMENT


            THIS FIFTH AMENDMENT TO REVOLVING CREDIT AGREEMENT (this "Fifth 
Amendment") is made and entered into as of the 21st day of February, 1997, 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, the 
Third Amendment to Revolving Credit Agreement dated as of March 29, 1996 and 
the Fourth Amendment to Revolving Credit Agreement dated as of October 1, 
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.

            Effective as of December 30, 1996, Section 8.2(a)(xi) of the 
Credit Agreement is hereby deleted in its entirety and the following 
substituted in place thereof:

            "and (ix) Indebtedness of (a) TSI Corporation to Finova 
            Technology Finance, Inc. (formerly Financing for Science 
            International, Inc. ("Finova")) 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., and of the Parent, Argus Research 
            Laboratories, Inc., TSI Mason Laboratories, Inc., TSI Redfield 
            Laboratories, Inc., and TSI Washington Laboratories, Inc. to 
            Finova pursuant to guaranties of such lease; and (b) the Parent 
            to Transamerica Business Credit Corporation ("TBCC") pursuant to 
            the terms of the Master Lease Agreement dated as of December 30, 
            1996 by and between TBCC and the Parent, and of TSI 

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            Corporation, BioDevelopment Laboratories, Inc., TSI Mason 
            Laboratories, Inc., TSI Washington Laboratories, Inc., TSI 
            Redfield Laboratories, Inc., and Argus Research Laboratories, 
            Inc. to TBCC pursuant to guaranties of such lease provided that 
            the aggregate outstanding amount of all such Indebtedness shall 
            not exceed $5,300,000."

            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 Fifth Amendment and the Credit Agreement 
shall hereafter be read and construed together as a 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 Fifth Amendment.  By executing this Fifth 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 Fifth Amendment.

            4.   GOVERNING LAW.

            THIS FIFTH 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 Fifth 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 Fifth 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 the date first set forth above.

                                     THE BORROWERS:

                                     GENZYME TRANSGENICS CORPORATION



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


                                     Address of the Parent:

                                     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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