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                                                            Exhibit 10.19


                               PURCHASE AGREEMENT

                                     between

                          GELTEX PHARMACEUTICALS, INC.

                                       and

                               GENZYME CORPORATION

                            dated as of June 17, 1997






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                                TABLE OF CONTENTS


ARTICLE  1.  PURCHASE AND SALE OF SHARES AND LLC INTEREST...................  1
    1.1.  Authorization.....................................................  1
    1.2.  Sale and Purchase of the Shares; Sale, Assignment and 
             Purchase of the LLC Interest; Payments.........................  2
    1.3.  Delivery of Certificates..........................................  2

ARTICLE 2.  REPRESENTATIONS AND WARRANTIES OF GELTEX........................  2
    2.1.  Organization and Qualification of GelTex..........................  2
    2.2.  Capitalization....................................................  2
    2.3.  RenaGel, Inc......................................................  3
    2.4.  RenaGel LLC.......................................................  3
    2.5.  Issuance and Sale of Shares; Sale and Assignment of 
             LLC Interest...................................................  4
    2.6.  Ownership of LLC Interest.........................................  4
    2.7.  Authority for Agreement...........................................  4
    2.8.  SEC Reports.......................................................  4
    2.9.  Financial Statements..............................................  4
    2.10.  Absence of Undisclosed GelTex Liabilities........................  5
    2.11.  No Material Adverse Change.......................................  5
    2.12.  Tax Matters......................................................  5
    2.13.  No Breach........................................................  6
    2.14.  Actions and Proceedings..........................................  7
    2.15.  Compliance with Laws.............................................  7
    2.16.  Brokerage........................................................  7
    2.17.  Full Disclosure..................................................  7

ARTICLE 3.  REPRESENTATIONS AND WARRANTIES OF GENZYME.......................  8
    3.1.  Authority for Agreement...........................................  8
    3.2.  Investment........................................................  8
    3.3.  Restrictions on Transferability; Legend...........................  8
    3.4.  Experience........................................................  8
    3.5.  Brokerage.........................................................  9

ARTICLE 4.  CONDITIONS PRECEDENT............................................  9
    4.1.  Certificates and Documents........................................  9
    4.2.  Opinion of Counsel................................................  9

ARTICLE 5.  MISCELLANEOUS................................................... 10
    5.1.  Assignment........................................................ 10
    5.2.  Severability...................................................... 10
    5.3.  Notices........................................................... 10
    5.4.  Applicable Law.................................................... 11



                                       (i)


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    5.5.  Entire Agreement.................................................. 11
    5.6.  Headings.......................................................... 11
    5.7.  Counterparts...................................................... 11



                                      (ii)


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                               PURCHASE AGREEMENT



        THIS PURCHASE AGREEMENT dated as of June 17, 1997 (the "AGREEMENT") is
made by and among GelTex Pharmaceuticals, Inc., a Delaware corporation having
its principal place of business at 303 Bear Hill Road, Waltham, Massachusetts
02154 ("GELTEX"), and Genzyme Corporation, a Massachusetts corporation having
its principal place of business at One Kendall Square, Cambridge, Massachusetts
02139 ("GENZYME"). Capitalized terms not defined herein shall have the
respective meanings ascribed to them in the Collaboration Agreement of even date
herewith (the "COLLABORATION AGREEMENT") by and among GelTex, Genzyme and
RenaGel LLC, a Delaware limited liability company having its principal place of
business at 303 Bear Hill Road, Waltham, Massachusetts 02154 ("RENAGEL LLC").
GelTex and Genzyme are sometimes referred to herein individually as a "PARTY"
and collectively as the "PARTIES."


                                 R E C I T A L S

        WHEREAS, GelTex, Genzyme and RenaGel LLC have entered into a
Collaboration Agreement for the development and commercialization of
Collaboration Products throughout the world (excluding the Chugai Territory);
and

        WHEREAS, in contemplation of such collaboration, GelTex has formed
RenaGel LLC and RenaGel, Inc., a Delaware corporation and a wholly-owned
subsidiary of GelTex ("RENAGEL, INC."), and has assigned one percent (1%) of its
interest in RenaGel LLC to RenaGel, Inc.; and

        WHEREAS, in connection with such collaboration, GelTex desires to issue
and sell to Genzyme and Genzyme desires to purchase from GelTex shares of its
Common Stock; and

        WHEREAS, in connection with the collaboration, GelTex desires to sell
and assign to Genzyme and Genzyme desires to purchase from GelTex a fifty
percent (50%) interest in RenaGel LLC.

        NOW THEREFORE, in consideration of the premises and of the covenants
herein contained, the Parties mutually agree as follows:


             ARTICLE 1. PURCHASE AND SALE OF SHARES AND LLC INTEREST

        1.1.    AUTHORIZATION. GelTex has duly authorized (a) the issuance and
sale by GelTex to Genzyme of 100,000 shares (the "SHARES") of GelTex's Common
Stock, $0.01 par value per share ("COMMON STOCK") and (b) the sale and
assignment by GelTex to Genzyme of a fifty percent (50%) interest in RenaGel LLC
(subject to adjustment pursuant to Section 4.2 of the Collaboration Agreement
and pursuant to the Operating Agreement of RenaGel LLC of even date herewith by
and among GelTex, Genzyme and RenaGel, Inc. (the "OPERATING AGREEMENT")) (the
"LLC INTEREST").






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        1.2.    SALE AND PURCHASE OF THE SHARES; SALE, ASSIGNMENT AND PURCHASE
OF THE LLC INTEREST; PAYMENTS. Concurrently with the execution and delivery of
this Agreement, GelTex hereby (a) issues and sells to Genzyme, and Genzyme
hereby purchases from GelTex, the Shares for an aggregate purchase price of Two
Million Five Hundred Thousand Dollars ($2,500,000) payable by Genzyme to GelTex
upon execution of this Agreement and (b) sells, assigns and transfers to
Genzyme, and Genzyme hereby purchases from GelTex the LLC Interest for an
aggregate purchase price of Twenty-Five Million Ten Dollars ($25,000,010)
payable as follows: (i) Ten Dollars ($10) payable by Genzyme to GelTex upon
execution of this Agreement; (ii) Fifteen Million Dollars ($15,000,000) payable
by Genzyme to GelTex within thirty (30) days following receipt by GelTex of FDA
approval of the NDA for RenaGel(R) non-absorbed phosphate binder ("RENAGEL(R)");
and (iii) Ten Million Dollars ($10,000,000) payable by Genzyme to GelTex on or
before the first anniversary of the date of receipt by GelTex of FDA approval of
the NDA for RenaGel(R). All of the aforementioned payments shall be made in
United States dollars by certified or bank check or wire transfer.

        1.3.    DELIVERY OF CERTIFICATES. On the date hereof, GelTex will
deliver to Genzyme one or more certificates representing the Shares registered
in the name of Genzyme.


               ARTICLE 2. REPRESENTATIONS AND WARRANTIES OF GELTEX

        In connection with the issuance and sale of the Shares and the sale and
assignment of the LLC Interest by GelTex to Genzyme, GelTex hereby makes the
following representations and warranties to Genzyme.

        2.1.    ORGANIZATION AND QUALIFICATION OF GELTEX. GelTex is a
corporation duly organized, validly existing and in good standing under the laws
of the State of Delaware and has full corporate power and lawful authority to
own, lease and operate its assets, properties and business and to carry on its
business as now being and as heretofore conducted. GelTex is qualified or
otherwise authorized to transact business as a foreign corporation in the
Commonwealth of Massachusetts, which is the only jurisdiction (in the United
States and outside of the United States) in which such qualification or
authorization is required by law and in which the failure to so qualify or be
authorized could have a material adverse effect on GelTex or its assets,
properties, business, operations or condition (financial or otherwise) (the
"BUSINESS OF GELTEX").

        2.2.    CAPITALIZATION.
                --------------

                (a)     GelTex is authorized to issue 50,000,000 shares of
Common Stock, of which 13,539,002 shares were issued and outstanding as of June
9, 1997 and 5,000,000 shares 


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of Preferred Stock, $0.01 par value per share, 500,000 of which have been
designated "Series A Junior Participating Preferred Stock" and none of which are
issued and outstanding. No other class of capital stock of GelTex is authorized
or outstanding. All of the issued and outstanding shares of GelTex's capital
stock are duly authorized and are validly issued, fully paid, nonassessable and
free of pre-emptive rights. None of the issued and outstanding shares have been
issued in violation of any federal or state law except for violations as would
not have a material adverse effect on the Business of GelTex.

                (b)     Options representing in the aggregate the right to
purchase 1,103,656 shares of Common Stock pursuant to GelTex's 1992 Equity
Incentive Plan and GelTex's 1995 Director Stock Option Plan and (ii) warrants
representing in the aggregate the right to purchase 11,400 shares of Common
Stock, are outstanding as of the date hereof.

                (c)     250,000 shares of Common Stock are reserved for issuance
under GelTex's 1995 Employee Stock Purchase Plan as of the date hereof.

                (d)     Except (i) as set forth in paragraphs (a), (b) and (c)
of this Section 2.2, or as a result of the exercise of outstanding options or
rights set forth therein, (ii) for certain rights of repurchase granted to
GelTex in connection with the exercise of certain unvested stock options and
(iii) for any Rights or shares that may be issued or purchased upon certain
circumstances described in the Rights Agreement by and between GelTex and
American Stock Transfer and Trust Company dated March 1, 1996, there are not, as
of the Effective Date, any other shares of GelTex capital stock authorized or
outstanding or any subscriptions, options, conversion or exchange rights,
warrants, repurchase or redemption agreements, or other agreements, commitments
or obligations of GelTex to issue, transfer, sell, repurchase or redeem any
shares of its capital stock or other securities of GelTex. To the best knowledge
of GelTex, there are no written stockholder agreements, voting trusts, proxies
or other agreements, instruments or understandings with respect to the voting of
the capital stock of GelTex.

        2.3.    RENAGEL, INC.. RenaGel, Inc. is a corporation duly organized,
validly existing and in good standing under the laws of the State of Delaware.
As of the date hereof, GelTex is the sole stockholder of RenaGel, Inc. and Mark
Skaletsky is the sole director of RenaGel, Inc.

        2.4.    RENAGEL LLC. RenaGel LLC is a limited liability company duly
organized, validly existing and in good standing under the laws of the State of
Delaware and has full corporate power and lawful authority to own, lease and
operate its assets, properties and business and to carry on its business as
contemplated by the Collaboration Agreement. Other than in the Commonwealth of
Massachusetts, RenaGel LLC is not required to be qualified or otherwise
authorized to transact business as a foreign limited liability company in any
jurisdiction (in the United States and outside of the United States) in which
such qualification or authorization is required by law and in which the failure
to so qualify or be authorized could have a material adverse effect on RenaGel
LLC or its assets, properties business,





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operations or condition (financial or otherwise) (the "BUSINESS OF RENAGEL
LLC"). The appropriate documents have been filed to qualify RenaGel LLC as a
foreign limited liability company in the Commonwealth of Massachusetts.
Immediately prior to the execution and delivery of this Agreement, GelTex is
the sole member of RenaGel LLC and GelTex and RenaGel, Inc. each hold a fifty
percent (50%) ownership interest in RenaGel LLC. GelTex has previously
exclusively licensed (subject to certain exceptions described in the
Collaboration Agreement) to RenaGel LLC all of its right, title and interest in
RenaGel(R) and other non-absorbed phosphate binders and technology relating
thereto, including without limitation all patents, know-how, trade secrets, and
pre-clinical and clinical data.

        2.5.    ISSUANCE AND SALE OF SHARES; SALE AND ASSIGNMENT OF LLC
INTEREST. The issuance and delivery of the Shares and the sale of the LLC
Interest by GelTex have been duly authorized and the Shares have been duly
reserved for issuance by all necessary corporate action on the part of GelTex,
and the Shares, when issued and paid for, will be duly and validly issued, fully
paid and non-assessable. Based in part on the representations made by Genzyme
set forth in Article 4 below, the offer, issuance and sale of the Shares and the
offer and sale of the LLC Interest pursuant to this Agreement are exempt from
registration under the Securities Act of 1933, as amended (the "SECURITIES
ACT"), and applicable state securities laws. GelTex has complied with all
applicable federal and state securities laws in connection with the offer,
issuance and sale of the Shares and the offer and sale of the LLC Interest.

        2.6.    OWNERSHIP OF LLC INTEREST. GelTex is the legal and beneficial
owner of all of the LLC Interest, free and clear of all liens, encumbrances,
restrictions and claims of all kinds. GelTex has full legal right, power and
authority to sell, assign, convey, transfer and deliver the LLC Interest to
Genzyme pursuant to this Agreement. The assignment by GelTex of the LLC
Interest, together with the execution of the Operating Agreement, pursuant to
the provisions hereof will transfer to Genzyme valid title to the LLC Interest,
free and clear of all liens, encumbrances, restrictions and claims of every kind
arising through GelTex.

        2.7.    AUTHORITY FOR AGREEMENT. The execution, delivery and performance
by GelTex of this Agreement has been duly authorized by all necessary corporate
action, and this Agreement has been duly executed and delivered by GelTex. This
Agreement constitutes the valid and binding obligation of GelTex enforceable
against GelTex in accordance with its terms, subject to applicable bankruptcy,
insolvency, moratorium and similar laws affecting the rights and remedies of
creditors generally and to general principles of equity.

        2.8.    SEC REPORTS. GelTex has previously delivered to Genzyme its (i)
Annual Report on Form 10-K for the year ended December 31, 1996 (the "GELTEX
10-K"), as filed with the Securities and Exchange Commission (the "SEC"), (ii)
all proxy statements relating to GelTex's meetings of stockholders held or
currently scheduled to be held since December 31,



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1996 and (iii) all other reports filed by GelTex with the SEC under the
Securities Exchange Act of 1934, as amended (the "EXCHANGE ACT") since December
31, 1996. As of their respective dates, such reports complied in all material
respects with applicable SEC requirements and did not contain any untrue
statement of a material fact or omit to state a material fact required to be
stated therein or necessary to make the statements therein, in light of the
circumstances under which they were made, not misleading. GelTex has timely
filed with the SEC all reports required to be filed under Section 13, 14 or
15(d) of the Exchange Act since December 31, 1996.

        2.9.    FINANCIAL STATEMENTS. The financial statements contained in the
GelTex 10-K and in GelTex's quarterly report on Form 10-Q for the quarter ended
March 31, 1997 (the "GELTEX 10-Q") have been prepared from, and are in
accordance with, the books and records of GelTex and present fairly, in all
material respects, the financial condition and results of operations of GelTex
as of and for the periods presented therein, all in conformity with United
States generally accepted accounting principles, consistently applied, except as
otherwise noted therein and subject (in the case of the unaudited financial
statements included in the GelTex 10-Q) to normal year-end adjustments, which
are not, in the aggregate, material.

        2.10.   ABSENCE OF UNDISCLOSED GELTEX LIABILITIES. As of December 31,
1996, GelTex had no material liabilities of any nature, whether accrued,
absolute, contingent or otherwise, (a) required to be reflected or disclosed on
the balance sheets dated December 31, 1996 (or the notes thereto) included in
the GelTex 10-K that were not adequately reflected or reserved against on such
balance sheets or (b) for the payment of any dividends, or any other
distribution, to the stockholders of GelTex. Except for certain capital
contributions and process development funding required pursuant to the Contract
Manufacturing Agreement and the Research Services Agreement by and between
GelTex and Dow Chemical and a payment of $1,500,000 required to be made to
Abbott Laboratories as a one time royalty payment (which GelTex and Abbott
Laboratories have agreed to re-characterize as a payment for FDA qualification
work), all of which have been previously disclosed to Genzyme, GelTex has no
liabilities, other than liabilities (i) adequately reflected or reserved against
on such balance sheet, (ii) reflected in GelTex's unaudited consolidated balance
sheets (or the notes thereto) included in the GelTex 10-Q, (iii) incurred since
December 31, 1996 in the ordinary course of business or (iv) that would not, in
the aggregate, have a material adverse effect on GelTex.

        2.11.   NO MATERIAL ADVERSE CHANGE. Since December 31, 1996, there has
not been any material adverse change in the assets, properties, business,
prospects, results of operations or financial condition of GelTex taken as a
whole, except for continuing losses from operations.

        2.12.   TAX MATTERS.
                -----------

                (a)     Except for any taxes that may be due upon the completion
of an on-going Massachusetts Department of Revenue sales tax audit, and except
for any taxes that



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may be due upon the filing of the 1996 Federal and state income tax returns
currently under extension, GelTex has paid or caused to be paid all federal,
state, county, local, foreign and other taxes, including without limitation
income taxes, estimated taxes, alternative minimum taxes, excise taxes, sales
taxes, use taxes, import duties, value-added taxes, gross receipts taxes,
franchise taxes, capital stock taxes, employment and payroll-related taxes,
withholding taxes, stamp taxes, transfer taxes, windfall profit taxes,
environmental taxes and property taxes, whether or not measured in whole or in
part by net income and all deficiencies, or other additions to such taxes and
interest, fines and penalties thereon (hereinafter, "TAXES" or, individually, a
"TAX") required to be paid by GelTex through the date hereof whether disputed or
not. The provisions for Taxes reflected in the financial statements referenced
in Section 2.8 above are adequate to cover any and all Tax liabilities of GelTex
in respect of its assets, properties, business and operations during the periods
covered by said financial statements and all prior periods. Except for its
knowledge of the on-going Massachusetts Department of Revenue sales tax audit,
GelTex does not know of any Tax deficiency or claim for additional Taxes or
interest thereon or penalties in connection therewith, asserted or threatened to
be asserted against GelTex by any taxing authority.

                (b)     GelTex has in accordance with applicable law timely
filed or received an extension for filing of all Tax reports or returns required
to be filed by it through the date hereof. Each of the Tax reports and returns
filed by GelTex correctly and accurately reflects the amount of its Tax
liability for such period and other required information. Except for its
knowledge of the on-going Massachusetts Department of Revenue sales tax audit,
there has not been any audit of any Tax return filed by GelTex and no audit of
any Tax return of GelTex is in progress and GelTex has not been notified by any
Tax authority that any such audit is contemplated or pending. No waiver or
agreement by GelTex is in force for the extension of time for the assessment or
payment of any Tax. No claim has ever been made by an authority in a
jurisdiction where GelTex does not file reports or returns that GelTex is or may
be subject to taxation by that jurisdiction. There are no security interests on
any of the assets of GelTex that arose in connection with any failure (or
alleged failure) to pay any Taxes. GelTex has never entered into a closing
agreement pursuant to Section 7121 of the Internal Revenue Code of 1986, as
amended (the "CODE").

                (c)     For purposes of this Agreement, all references to
sections of the Code shall include any predecessor provisions to such sections
and any similar provisions of federal, state, local or foreign law.

        2.13.   NO BREACH. The execution, delivery and performance of this
Agreement, the Collaboration Agreement and the Operating Agreement and the
consummation of the transactions contemplated hereby and thereby will not: (i)
violate any provision of the Restated Certificate of Incorporation or By-laws of
GelTex, the Certificate of Incorporation or By-laws of RenaGel, Inc. or the
Certificate of Formation of RenaGel LLC; (ii) violate, conflict with or result
in the breach of any of the terms or conditions of, result in modification of
the effect of, or otherwise give any other contracting party the right to
terminate, or constitute (or with notice or lapse of time or both constitute) a
default under, any material instrument, contract or 




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other agreement to which GelTex is a party or to which any of the assets or
properties of GelTex or RenaGel LLC may be bound or subject; (iii) violate any
order, judgment, injunction, award or decree of any court, arbitrator or
governmental or regulatory body against, or binding upon, GelTex, RenaGel, Inc.
or RenaGel LLC or upon the securities, properties, assets or business of GelTex,
RenaGel, Inc. or RenaGel LLC; (iv) violate any statute, law or regulation of any
jurisdiction as such statute, law or regulation relates to GelTex, RenaGel, Inc.
or RenaGel LLC or to the securities, properties, assets or business of GelTex,
RenaGel, Inc. or RenaGel LLC, respectively; (v) require the approval, consent or
authorization of, or registration or filing with, any foreign, federal, state,
local or other governmental or regulatory body or the approval, consent, waiver
or notification of any stockholder, creditor, lessor or other non-governmental
and non-regulatory persons; or (vi) result in the creation of any lien or other
encumbrance on the assets or properties of GelTex, RenaGel, Inc. or RenaGel LLC,
excluding from clauses (ii) - (vi) such matters as would not in the aggregate
have a material adverse effect on the Businesses of GelTex, RenaGel, Inc. or
RenaGel LLC or upon the transactions contemplated hereby or by the Collaboration
Agreement or the Operating Agreement. Prior to the date hereof, neither RenaGel
LLC nor RenaGel, Inc. was a party to any contracts or agreements with a Third
Party.

        2.14.   ACTIONS AND PROCEEDINGS. There are no outstanding orders,
judgments, injunctions, awards or decrees of any court, governmental or
regulatory body or arbitration tribunal against GelTex or RenaGel LLC or
affecting any of their respective properties or rights. There are no actions,
suits or claims or legal, administrative or arbitral proceedings or, to the best
knowledge of GelTex, investigations (whether or not the defense thereof or
liabilities in respect thereof are covered by insurance) pending or, to the best
knowledge of GelTex, threatened against GelTex or RenaGel LLC or affecting any
of their respective properties or rights. To the best knowledge of GelTex, there
is no fact, event or circumstance that may give rise to any suit, action, claim,
investigation or proceeding that individually or in the aggregate could have a
material adverse effect upon the transactions contemplated hereby or upon the
Businesses of GelTex or RenaGel LLC. References in this Agreement to the
Business of RenaGel, Inc. or RenaGel LLC mean the assets, properties, business,
operations or condition (financial or otherwise) of RenaGel, Inc. or RenaGel
LLC, as applicable.

        2.15.   COMPLIANCE WITH LAWS. Neither GelTex nor RenaGel LLC is in
violation of any statute, law, rule or regulation, or in default with respect to
any judgment, writ, injunction, decree, rule or regulation of any court or
governmental agency or instrumentality specifically naming GelTex or RenaGel
LLC, including without limitation laws relating to environmental protection,
except for such violations or defaults which do not, individually or in the
aggregate, materially and adversely affect the Businesses of GelTex or RenaGel
LLC.

        2.16.   BROKERAGE. No broker, finder, agent or similar intermediary has
acted on behalf of GelTex in connection with this Agreement or the transactions
contemplated hereby, and there are no brokerage commissions, finders fees or
similar fees or commissions payable in connection therewith based on any
agreement, arrangement or understanding with GelTex or any action taken by
either GelTex or RenaGel, Inc.



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<PAGE>   11
        2.17.   FULL DISCLOSURE. No representation or warranty of GelTex
contained in this Agreement nor the GelTex 10-K or the GelTex 10-Q, taken as a
whole as of the date hereof, (i) contains an untrue statement of a material fact
or omits to state a material fact (i) required to be stated therein or necessary
to make the statements made, in the context in which made, not false or
misleading or (ii) omits to state a material fact that materially adversely
affects, or (in the reasonable business judgment of GelTex based on facts of
which it has knowledge) is likely to materially adversely affect the Business of
GelTex, and, to the best knowledge of GelTex, no other document or paper
furnished by or on behalf of GelTex to Genzyme (or any of its agents) pursuant
to this Agreement or in connection with the transactions contemplated hereby,
taken as a whole as of the date hereof together with the representations and
warranties of GelTex contained in this Agreement and the GelTex 10-K and GelTex
10-Q contains an untrue statement of a material fact. There is no fact known to
GelTex that has not been disclosed to Genzyme in this Agreement or otherwise
that materially adversely affects, or (in the reasonable business judgment of
GelTex based on facts of which it has knowledge) is likely to materially
adversely affect the Business of RenaGel LLC.


              ARTICLE 3. REPRESENTATIONS AND WARRANTIES OF GENZYME

        In connection with the purchase by Genzyme of the Shares and the LLC
Interest from GelTex, Genzyme hereby makes the following representations and
warranties to GelTex.

        3.1.    AUTHORITY FOR AGREEMENT. The execution, delivery and performance
by Genzyme of this Agreement has been duly authorized by all necessary corporate
action, and this Agreement has been duly executed and delivered by Genzyme. This
Agreement constitutes the valid and binding obligation of Genzyme enforceable
against Genzyme in accordance with its terms, subject to applicable bankruptcy,
insolvency, moratorium and similar laws affecting the rights and remedies of
creditors generally and to general principles of equity.

        3.2.    INVESTMENT. Genzyme is acquiring the Shares and the LLC Interest
solely for its own account, for investment and not with a view to, or for sale
in connection with, any distribution thereof, nor with any present intention of
distributing or selling the same; Genzyme does not have any present plans to
enter into any contract, undertaking, agreement or arrangement relating thereto.

        3.3.    RESTRICTIONS ON TRANSFERABILITY; LEGEND. Genzyme understands
that neither the Shares nor the LLC Interest has been registered under the
Securities Act or under the securities laws of any state or other jurisdiction
in reliance upon exemptions thereunder. Genzyme acknowledges and is aware that
neither the Shares nor the LLC Interest can be resold unless the Shares or the
LLC Interest, respectively, are registered under the Securities Act and any
applicable securities law of any state or other jurisdiction, or an exemption
from registration is available, and that it has no rights to require that the
Shares or the LLC Interest be registered under the Securities Act or any state
securities laws.


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<PAGE>   12
        3.4.    EXPERIENCE. Genzyme has carefully reviewed (i) the
representations concerning GelTex and RenaGel LLC contained in this Agreement,
(ii) the GelTex 10-K and (iii) the GelTex 10-Q, and has had the opportunity to
make detailed inquiry concerning GelTex, RenaGel LLC and their respective
businesses and personnel. The officers of GelTex have made available to Genzyme
any and all written information which it has requested and have answered to
Genzyme's satisfaction all inquiries made by Genzyme. Genzyme has adequate net
worth and means of providing for its current needs and contingencies to sustain
a complete loss of its investment in GelTex and RenaGel LLC. Genzyme's overall
commitments to investments which are not readily marketable is not
disproportionate to its net worth, and Genzyme's investment in the Shares and
the LLC Interest will not cause such overall commitment to become excessive.
Genzyme has sufficient knowledge and experience to evaluate the risk of its
investment in each of GelTex and RenaGel LLC.

        3.5.    BROKERAGE. No broker, finder, agent or similar intermediary has
acted on behalf of Genzyme in connection with this Agreement or the transactions
contemplated hereby, and there are no brokerage commissions, finders fees or
similar fees or commissions payable in connection therewith based on any
agreement, arrangement or understanding with Genzyme, or any action taken by it.


                         ARTICLE 4. CONDITIONS PRECEDENT

        4.1.    CERTIFICATES AND DOCUMENTS. Contemporaneously with the execution
of this Agreement, GelTex shall deliver to Genzyme:

        (a)     The Restated Certificate of Incorporation of GelTex, certified
by the Secretary of State of Delaware as of the most recent practicable date;

        (b)     The Certificate of Formation of RenaGel LLC, certified by the
Secretary of the State of Delaware as of the most recent practicable date;

        (c)     The Certificate of Incorporation of RenaGel, Inc., certified by
the Secretary of the State of Delaware as of the most recent practicable date;

        (d)     Certificate, as of the most recent practicable date, as to the
corporate good standing and legal existence of GelTex issued by the Secretary of
State of Delaware and the Secretary of the Commonwealth of Massachusetts, each
confirming such good standing and legal existence as of such date, together with
a facsimile from the Secretary of State of Delaware confirming the legal
existence of GelTex as of the most recent practicable date;

        (e)     Facsimile from the Secretary of State of Delaware dated the most
recent practicable date confirming the corporate good standing and legal
existence of RenaGel LLC on the most recent practicable date;




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<PAGE>   13


        (f)     By-laws of GelTex, certified by its Secretary as of the most
recent practicable date; and

        (g)     Resolutions of the Board of Directors of GelTex authorizing and
approving all matters in connection with this Agreement and the transactions
contemplated hereby, certified by the Secretary of GelTex as of the most recent
practicable date.

        4.2.    OPINION OF COUNSEL. Contemporaneously with the execution of this
Agreement, GelTex shall deliver to Genzyme an opinion of counsel to GelTex,
dated as of the date hereof and addressed to Genzyme, in a form reasonably
acceptable to Genzyme and its counsel.


                            ARTICLE 5. MISCELLANEOUS

        5.1.    ASSIGNMENT. This Agreement may not be assigned or otherwise
transferred by any Party without the consent of the other Party; provided,
however, that either Party may, without such consent, assign its rights and
obligations under this Agreement (a) in connection with a corporate
reorganization, to any member of an affiliated group, all or substantially all
of the equity interest of which is owned and controlled by such Party or its
direct or indirect parent corporation or (b) in connection with a merger,
consolidation or sale of substantially all of such Party's assets to an
unrelated Third Party; provided, however, that such Party's rights and
obligations under this Agreement shall be assumed by its successor in interest
in any such transaction and shall not be transferred separate from all or
substantially all of its other business assets, including those business assets
that are the subject of this Agreement. Any purported assignment in violation of
the preceding sentence shall be void. Any permitted assignee shall assume all
obligations of its assignor under this Agreement in writing.

        5.2.    SEVERABILITY. Each Party hereby agrees that it does not intend
to violate any public policy, statutory or common laws, rules, regulations,
treaty or decision of any government agency or executive body thereof of any
country or community or association of countries. Should one or more provisions
of this Agreement be or become invalid, the Parties hereto shall substitute, by
mutual consent, valid provisions for such invalid provisions, which valid
provisions in their economic effect are sufficiently similar to the invalid
provisions that it can be reasonably assumed that the Parties would have entered
into this Agreement with such valid provisions. In case such valid provisions
cannot be agreed upon, the invalidity of one or several provisions of this
Agreement shall not affect the validity of this Agreement as a whole, unless the
invalid provisions are of such essential importance to this Agreement that it is
to be reasonably assumed that the Parties would not have entered into this
Agreement without the invalid provisions.



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        5.3.    NOTICES. Any consent, notice or report required or permitted to
be given or made under this Agreement by one of the Parties hereto to the other
shall be in writing, delivered personally or by facsimile (and promptly
confirmed by personal delivery or courier) or courier, postage prepaid (where
applicable), addressed to such other Party at its address indicated below, or to
such other address as the addressee shall have last furnished in writing to the
addressor in accordance with this Section 5.3, and shall be effective upon
receipt by the addressee.

         If to                      GelTex Pharmaceuticals, Inc.
         GelTex:                    303 Bear Hill Road
                                    Waltham, Massachusetts 02154
                                    Attention: President
                                    Facsimile: (617) 672-5822

         with a copy to:            GelTex Pharmaceuticals, Inc.
                                    303 Bear Hill Road
                                    Waltham, Massachusetts 02154
                                    Attention: Corporate Counsel
                                    Facsimile: (617) 672-5822

         If to                      Genzyme Corporation
         Genzyme:                   One Kendall Square
                                    Cambridge, Massachusetts 02139
                                    Attention: President
                                    Facsimile: (617) 374-7423

         with a copy to:            Genzyme Corporation
                                    One Kendall Square
                                    Cambridge, Massachusetts 02139
                                    Attention: Chief Legal Counsel
                                    Facsimile: (617) 252-7553

        5.4.    APPLICABLE LAW. This Agreement shall be governed by and
construed in accordance with the laws of the Commonwealth of Massachusetts.

        5.5.    ENTIRE AGREEMENT. This Agreement together with the Collaboration
Agreement and the Operating Agreement contain the entire understanding of the
Parties with respect to the subject matter hereof. All express or implied
agreements and understandings, either oral or written, heretofore made are
expressly merged in and made a part of this Agreement. This Agreement may be
amended, or any term hereof modified, only by a written instrument duly executed
by the Parties. Each of the Parties hereby acknowledges that this Agreement, the
Collaboration Agreement and the Operating Agreement are each the result of
mutual negotiation and therefore any ambiguity in their respective terms shall
not be construed against the drafting Party.


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        5.6.    HEADINGS. The captions to the several Articles and Sections
 hereof are not a part of this Agreement, but are merely guides or labels to
assist in locating and reading the several Articles and Sections hereof.

        5.7.    COUNTERPARTS. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.


        IN WITNESS WHEREOF, the Parties have executed this Agreement as of the
date first set forth above.


                                   GELTEX PHARMACEUTICALS, INC.


                                   By: /s/  Mark Skaletsky
                                       --------------------------------------
                                   Title: President and CEO


                                   GENZYME CORPORATION


                                   By: /s/  Peter Wirth
                                       --------------------------------------
                                   Title: Executive Vice President
                                   



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