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                             SUPPLEMENTAL AGREEMENT


         This Supplemental Agreement is made as of December 15, 2000 among
Seagull Merger Corporation, a Massachusetts corporation ("Seagull"), Genzyme
Corporation, a Massachusetts corporation ("Genzyme"), and UBS Warburg LLC
("Warburg"), a Delaware limited liability company.

                                    RECITALS

         A.       Biomatrix, Inc. ("Biomatrix") has issued a 6.9% Convertible
Subordinated Note due May 14, 2003 (the "Convertible Note") in favor of
Warburg.

         B.       Biomatrix has executed an Agreement and Plan of Merger
dated as of March 6, 2000 (as amended, the "Merger Agreement") with Genzyme
and Seagull pursuant to which Biomatrix will merge with and into Seagull, and
Seagull shall continue as the surviving entity (the "Merger").

         C.       In connection with the Merger, and in accordance with the
terms of the Convertible Note, Seagull has agreed to assume the obligations
under the Convertible Note.

                                    AGREEMENT

1.       ASSUMPTION OF CONVERTIBLE NOTE.

         As provided in Section 5 of the Convertible Note, effective as of
the effective time of the Merger, Seagull, as the entity surviving the
Merger, hereby assumes the due and punctual payment of the principal of (and
premium, if any) and interest on the Convertible Note and the performance or
observance of every covenant of the Convertible Note on the part of the
Company (as defined in the Convertible Note) to be performed or observed,
except as follows:

         (a)      In Section 1, the words "Company Stock" shall be deleted
and replaced with the words "Common Stock";

         (b)      The first paragraph of Section 2(a) of the Convertible Note
shall be deleted and replaced in its entirety with the following:

                "2. CONVERSION. (a) The holder of this Security is entitled at
       any time on or after May 14, 1999 and before the close of business on
       May 14, 2003 (or, in case this Security or a portion hereof is called
       for redemption or the holder hereof has exercised his right to require
       the Company to repurchase this Security or a portion hereof, then in
       respect of this Security or such portion hereof, as the case may be,
       until and including, but (unless the Company defaults in making the
       payment due upon redemption or repurchase, as the case may be) not
       after, the close of business on the redemption date or the Repurchase
       Date, as the case may be) to convert this Security (or any portion of
       the principal amount hereof that is an integral multiple of $1,000),
       into (1) fully paid and nonassesable shares (calculated as to each
       conversion to the nearest 1/100 of a share) of

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       Common Stock at the rate of 35.81 shares of Common Stock (the
       "Conversion Rate") for each $1,000 principal amount of Security (or
       at the current adjusted rate if an adjustment has been made as
       provided below) and (2) cash at the rate of $525.03 (the "Cash Rate")
       for each $1,000 principal amount of Security by surrender of this
       Security, duly endorsed or assigned to the Company or in blank to the
       Company at the Designated Office, accompanied by written notice to
       the Company that the holder hereof elects to convert this Security
       (or if less than the entire principal amount hereof is to be
       converted, specifying the portion hereof to be converted). Upon
       surrender of this Security for conversion, the holder will be
       entitled to receive the interest accruing on the principal amount of
       this Security then being converted from the interest payment date
       next preceding the date of such conversion to such date of
       conversion. No payment or adjustment is to be made on conversion for
       dividends on the Common Stock issued on conversion hereof. No
       fractions of shares or scrip representing fractions of shares will be
       issued on conversion, but instead of any fractional interest, the
       Company shall pay a cash adjustment, computed on the basis of the
       Closing Price of the Common Stock on the date of conversion, or, at
       its option, the Company shall round up to the next higher whole
       share."

         The Conversion Rate and Cash Rate specified in the replacement
paragraph above are based upon the standard consideration that non-electing
common stockholders of Biomatrix are entitled to receive in connection with
the Merger, as set forth in the Merger Agreement. If the standard
consideration is adjusted in connection with the closing of the Merger, then
(1) an appropriate and proportionate adjustment shall be made to the
Conversion Rate and Cash Rate and (2) the Conversion Rate and Cash Rate
specified in the replacement paragraph above shall mean the Conversion Rate
and Cash Rate as so adjusted;

         (c) In Sections 2(b) through (j), 3(b)(5), 3(g), 3(i) and 3(j)(2),
each instance of the term "the Company" shall be replaced with "Genzyme
Corporation";

         (d) In Section 2(i), the parenthetical clause, "(it being understood
that the Company shall not be required to register the Common Stock issuable
on conversion hereof under the Securities Act, except pursuant to the
Registration Rights Agreement between the Company and the initial holder of
this Security)" shall be deleted and replaced in its entirety with the
following:

       "(it being understood that the Company shall not be required to
       register the Common Stock issuable on conversion hereof under the
       Securities Act, except pursuant to the Supplemental Agreement among the
       Company, the initial holder of this Security and the other parties
       thereto)."

         (e) In Section 2(j), the definition of "Common Stock" shall be deleted
and replaced in its entirety with the following:

                " "Common Stock" means the Genzyme Biosurgery Division Common
       Stock, par value $0.01 per share, of Genzyme Corporation together with
       the associated GBS Stock Purchase Rights. Subject to the provisions of
       Section 2(h), shares issuable on conversion or repurchase of this
       Security shall include only shares of Genzyme Biosurgery Division
       Common Stock and the associated GBS Stock Purchase Rights or shares of
       any class or

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       classes of common stock resulting from any reclassification or
       reclassifications thereof; PROVIDED, HOWEVER, that if at any time
       there shall be more than one such resulting class, the shares so
       issuable on conversion of this Security shall include shares of all
       such classes, and the shares of each such class then so issuable
       shall be substantially in the proportion which the total number of
       shares of such class resulting from all such reclassifications bears
       to the total number of shares of all such classes resulting from all
       such reclassifications.";

         (f)      In Section 2(j), the definition of "Conversion Price" shall
be deleted and replaced in its entirety with the following:

                " "Conversion Price" on any day means an amount equal to the
       quotient determined by dividing (1) the difference obtained by
       subtracting the Cash Rate from $1,000 by (2) the Conversion Rate in
       effect on such day.";

         (g)      To add the following subsection after Section 2(j):

                "(k) In case Genzyme Corporation shall redeem or exchange
       shares of its Common Stock pursuant to the provisions governing
       redemption or exchange of its Common Stock as set forth in its Amended
       and Restated Articles of Organization, as amended, the holder of this
       Security shall have the right thereafter, during the period this
       Security shall be convertible as specified in Section 2(a), to convert
       this Security only into the kind and amount of securities, cash and
       other property receivable upon such redemption or exchange by a holder
       of the number of shares of Common Stock into which this Security might
       have been converted immediately prior to such redemption or exchange.";

         (h)      In Section 3(j)(2), each instance of the defined term
"Common Stock" shall be replaced with "capital stock of the Company";

         (i)      Section 4(a)(2) shall be deleted and replaced in its
entirety with the following:

                "(2) default by the Company or Genzyme Corporation in the
       performance of their respective obligations in respect of any
       conversion of this Security (or any portion hereof) in accordance with
       Section 2; or".

2.       ASSUMPTION OF REGISTRATION RIGHTS AGREEMENT.

         Effective as of the effective time of the Merger, (1) Warburg
releases Seagull from any and all obligations under the Registration Rights
Agreement dated as of May 14, 1998 by and between Biomatrix and Warburg (the
"Registration Rights Agreement"), (2) Warburg agrees not to sell any
securities pursuant to the prospectus which forms a part of the Registration
Statement on Form S-3 (Registration No. 333-77367) previously filed by
Biomatrix and (3), except for the following modifications, Genzyme assumes
the obligations and is entitled to the rights under the Registration Rights
Agreement:

         (a)      The defined term "the Company" shall mean Genzyme
Corporation, except in those instances where the term "the Company" refers to
the issuer of or the obligor on the

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Convertible Note, in which instance the term "the Company" shall mean Seagull;

         (b)      The defined term "Common Stock" shall mean the Genzyme
Biosurgery Division Common Stock, par value $0.01 per share, of Genzyme
Corporation;

         (c)      The second sentence of Section 2(a) shall be deleted and
replaced in its entirety with the following:

       "The Company agrees to use its reasonable best efforts (1) to cause the
       Shelf Registration to become or be declared effective as soon as
       practicable following the Company's filing with the SEC of the
       financial statements, pro forma financial information and exhibits
       required by Item 7 of Current Report on Form 8-K promulgated under the
       Securities Exchange Act of 1934, as amended, with respect to the
       Company's acquisitions of GelTex Pharmaceuticals, Inc. and Biomatrix
       and (2) to keep such Shelf Registration continuously effective for a
       period ending on the earliest to occur of (i) the 181st day following
       conversion of the Convertible Note into shares of Common Stock, (ii)
       notification to the Company by the Purchaser that it has sold all
       Shares issuable upon conversion of the Convertible Note so owned by it,
       (iii) such time as the Purchaser may sell all of such shares pursuant
       to Rule 144(k) (or any successor provision) under the Securities Act or
       (iv) the close of business on May 14, 2003, unless the Purchaser
       properly converts the Convertible Note into shares of Common Stock on
       May 14, 2003, in which case this subclause (iv) shall be deemed to
       refer to the close of business on November 10, 2003.";

         (d)      The text of Sections 3(a)(iii), (xii) and (xiv) shall be
deleted and replaced in their entirety with the following:

                  "[Removed and Reserved.]";

         (e)      Any reference in the Registration Rights Agreement to the
parties referred to in Section 3(a)(iii) shall mean the Purchaser and counsel
for the Purchaser;

         (f)      In Section 7(c), the notice information shall be deleted
and replaced in its entirety with the following:

       "If to the Company, to:

       Genzyme Corporation
       One Kendall Square
       Cambridge, Massachusetts 02139

       Attention:  Earl M. Collier, Jr.
       Telephone Number:  (617) 252-7500
       Facsimile Transmission Number:  (617) 252-7600

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       with a copy to:

       Palmer & Dodge LLP
       One Beacon Street
       Boston, Massachusetts 02108

       Attention:  Paul Kinsella
       Telephone Number:  (617) 573-0100
       Facsimile Transmission Number:  (617) 227-4420

       If to UBS Warburg LLC to:

       UBS Warburg LLC
       677 Washington Blvd.
       Stamford, CT 06901

       Attention:  Convertible Trading
       Telephone Number:  (203) 719-8510
       Facsimile Transmission Number:  (203) 719-8499

       Attention:  Operations - Corporate Action Department
       Telephone Number:  (203) 719-7644
       Facsimile Transmission Number:  (203) 719-0795

       Attention:  Legal Affairs (Equities Group)
       Telephone Number:  (203) 719-5427
       Facsimile Transmission Number:  (203) 719-0680"; and

         (g)      Notwithstanding anything to the contrary in the Registration
Rights Agreement, Genzyme shall not be required to enter into any underwriting
agreements, engagement letters, agency agreements or similar agreements in
connection with Warburg's disposition of the Shares (as defined in the
Registration Rights Agreement).

3.       PAYMENT ON THE NOTE.

         Notwithstanding anything to the contrary in this Supplemental
Agreement, the Convertible Note or the Registration Rights Agreement, Genzyme
shall not be liable to pay any amounts owed under the Convertible Note,
including without limitation principal of, premium on or interest on the
Convertible Note, and Warburg shall look only to Seagull for payment of
amounts owed under the Convertible Note.

4.       MISCELLANEOUS.

         This Supplemental Agreement shall be governed by and construed in
accordance with the laws of the State of New York, United States of America.

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<PAGE>

         IN WITNESS WHEREOF, the undersigned has caused this Supplemental
Agreement to be executed as of the date set forth above.

                       SEAGULL MERGER CORPORATION



                       By:      /s/ Earl M. Collier, Jr.
                                --------------------------------------------
                                Earl M. Collier, Jr.
                                President


                       GENZYME CORPORATION



                       By:      /s/ Earl M. Collier, Jr.
                                --------------------------------------------
                                Earl M. Collier, Jr.
                                Executive Vice President



                       UBS WARBURG LLC



                       By:      /s/ Richard A. Simpson
                                --------------------------------------------
                                Name: Richard A. Simpson
                                Title: Executive Director, Convertibles



                       By:      /s/ Nick Ogurtsov
                                --------------------------------------------
                                Name: Nick Ogurtsov
                                Title: Director, Equities



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