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                                                              EXHIBIT 10.52.2


                               PURCHASE AGREEMENT

                                     between

                         GENZYME TRANSGENICS CORPORATION

                                       and

                               GENZYME CORPORATION

                           dated as of January 1, 1998


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                                TABLE OF CONTENTS

<TABLE>
<S>                                                                         <C>
ARTICLE 1. PURCHASE AND SALE LLC INTEREST            
     1.1.  Authorization                                                     1
     1.2.  Sale, Assignment and Purchase of the LLC Interest; Payments       1

ARTICLE 2.  REPRESENTATIONS AND WARRANTIES OF GTC                            2
     2.1.  Organization and Qualification of GTC                             2
     2.2.  ATIII LLC                                                         2
     2.3.  Ownership of LLC Interest                                         2
     2.4.  Authority                                                         3
     2.5.  Offer and Sale of LLC Interest                                    3
     2.6.  No Breach                                                         3
     2.7.  Actions and Proceedings                                           3
     2.8.  Compliance with Laws                                              4
     2.9.  Brokerage                                                         4
     2.10.  Full Disclosure                                                  4

ARTICLE 3.  REPRESENTATIONS AND WARRANTIES OF GENZYME                        4
     3.1.  Authority for Agreement                                           4
     3.2.  Investment                                                        4
     3.3.  Restrictions on Transferability                                   5
     3.4.  Experience                                                        5
     3.5.  Brokerage                                                         5

ARTICLE 4.  CONDITION PRECEDENT                                              5
     4.1.  Clerk's Certificate                                               5
                                                                             5

ARTICLE 5.  MISCELLANEOUS                                                    5
     5.1.  Assignment                                                        5
     5.2.  Severability                                                      6
     5.3.  Notices                                                           6
     5.4.  Applicable Law                                                    7
     5.5.  Entire Agreement                                                  7
     5.6.  Headings                                                          7
     5.7.  Counterparts                                                      7
</TABLE>


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                               PURCHASE AGREEMENT



     THIS PURCHASE AGREEMENT dated as of January 1, 1998 (the "Agreement") is
made by and among Genzyme Transgenics Corporation, a Massachusetts corporation
having its principal place of business at Five Mountain Road, Framingham,
Massachusetts 01701 ("GTC"), 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
GTC, Genzyme and ATIII LLC, a Delaware limited liability company having its
principal place of business at One Kendall Square, Cambridge, Massachusetts
02139 ("ATIII LLC"). GTC 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, GTC, Genzyme and ATIII LLC have entered into a Collaboration
Agreement for the development and commercialization of Collaboration Products
throughout the world (excluding the SMIG Territory); and

     WHEREAS, in contemplation of such collaboration, GTC and Genzyme have
formed ATIII LLC and own a * percent * and * percent * interest in ATIII LLC,
respectively;

     WHEREAS, in connection with the collaboration, GTC desires to sell and
assign to Genzyme and Genzyme desires to purchase from GTC a * percent *
interest in ATIII 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 LLC INTEREST

     1.1 Authorization. GTC has duly authorized the sale and assignment by GTC
to Genzyme of a * percent (*%) interest in ATIII LLC (subject to adjustment
pursuant to Section 4.1.5 of the Collaboration Agreement and pursuant to the
Operating Agreement of ATIII LLC of even date herewith (the "Operating
Agreement") by and between GTC and Genzyme (the "LLC Interest").

     1.2 Sale, Assignment and Purchase of the LLC Interest; Payments.
Concurrently with the execution and delivery of this Agreement, GTC hereby
sells, assigns and transfers.

---------------------
* Confidential treatment for indicated portions respectfully requested.


                                       1

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to Genzyme,  and  Genzyme  hereby  purchases  from GTC the LLC  Interest  for an
aggregate  purchase price of * dollars ($*) payable as follows:  (i) ten dollars
($10) payable by Genzyme to GTC upon execution of this Agreement; (ii) * dollars
($*)  after  ($*) and (iii)  ($*)  dollars  * upon *. All of the  aforementioned
payments  shall be made in United States dollars by certified bank check or wire
transfer.


     ARTICLE 2. REPRESENTATIONS AND WARRANTIES OF GTC

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

     2.1 Organization and Qualification of GTC. GTC is a corporation duly
organized, validly existing and in good standing under the laws of the
Commonwealth of Massachusetts 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. GTC is qualified or
otherwise authorized to transact business as a foreign corporation in each
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 GTC or
its assets, properties, business, operations or condition (financial or
otherwise) (the "Business of GTC").

     2.2 ATIII LLC. ATIII 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. The appropriate documents have been filed to
qualify ATIII LLC as a foreign limited liability company in the Commonwealth of
Massachusetts. Immediately prior to the execution and delivery of this
Agreement, GTC and Genzyme are the only members of ATIII LLC and GTC and Genzyme
hold a * percent (*%) and * percent (*%) interest in ATIII LLC, respectively.
GTC has previously exclusively licensed (subject to certain exceptions described
in the Collaboration Agreement) to ATIII LLC all of its right, title and
interest in tgATIII and technology relating thereto in the Territory, including
without limitation all patents, know-how, trade secrets, and preclinical and
clinical data.

     2.3 Ownership of LLC Interest. GTC 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. GTC 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 GTC of the LLC Interest, together with the
execution of the Operating Agreement, pursuant to the provisions --------
A*Confidential treatment for indicated portions respectfully requested.

---------------------
* Confidential treatment for indicated portions respectfully requested.


                                        2

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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 GTC.

     2.4 Authority. The sale of the LLC Interest by GTC and the execution,
delivery and performance by GTC of this Agreement has been duly authorized by
all necessary corporate action, and this Agreement has been duly executed and
delivered by GTC. This Agreement constitutes the valid and binding obligation of
GTC enforceable against GTC 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.

     2.5 Offer and Sale of LLC Interest. Based in part on the representations
made by Genzyme set forth in Article 3 below, the offer and sale of the LLC
Interest pursuant to this Agreement is exempt from registration under the
Securities Act of 1933, as amended (the "Securities Act"), and applicable state
securities laws. GTC has complied with all applicable federal and state
securities laws in connection with the offer and sale of the LLC Interest.

     2.6 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 Articles of Organization or By-laws of GTC or the Certificate
of Formation of ATIII 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 other agreement to which GTC is a party or
to which any of the assets or properties of GTC or ATIII 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,
GTC or ATIII LLC or upon the securities, properties, assets or business of GTC
or ATIII LLC; (iv) violate any statute, law or regulation of any jurisdiction as
such statute, law or regulation relates to GTC or ATIII LLC or to the
securities, properties, assets or business of GTC or ATIII 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 GTC or ATIII LLC, excluding from clauses (ii) - (vi) such matters
as would not in the aggregate have a material adverse effect on the Businesses
of GTC or ATIII LLC or upon the transactions contemplated hereby or by the
Collaboration Agreement or the Operating Agreement. Prior to the date hereof,
ATIII LLC was not a party to any contracts or agreements with a Third Party.
References in this Agreement to the Business of ATIII LLC mean the assets,
properties, business, operations or condition (financial or otherwise) of ATIII
LLC.

     2.7 Actions and Proceedings. There are no outstanding orders, judgments,

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injunctions, awards or decrees of any court, governmental or regulatory body or
arbitration tribunal against GTC or ATIII 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 GTC,
investigations (whether or not the defense thereof or liabilities in respect
thereof are covered by insurance) pending or, to the best knowledge of GTC,
threatened against GTC or ATIII LLC or affecting any of their respective
properties or rights. To the best knowledge of GTC, 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 GTC
or ATIII LLC.

     2.8 Compliance with Laws. Neither GTC nor ATIII 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 GTC or ATIII 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 GTC or ATIII LLC.

     2.9 Brokerage. No broker, finder, agent or similar intermediary has acted
on behalf of GTC 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 GTC or any action taken by GTC.

     2.10 Full Disclosure. No representation or warranty of GTC contained in
this Agreement (i) contains an untrue statement of a material fact or omits to
state a material fact 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 GTC based on facts of which it has knowledge) is
likely to materially adversely affect the Business of GTC, and, to the best
knowledge of GTC, no other document or paper furnished by or on behalf of GTC 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 GTC contained in this
Agreement contains an untrue statement of a material fact. There is no fact
known to GTC that has not been disclosed to Genzyme in this Agreement or
otherwise that materially adversely affects, or (in the reasonable business
judgment of GTC based on facts of which it has knowledge) is likely to
materially adversely affect the Business of ATIII LLC.


               ARTICLE 3 REPRESENTATIONS AND WARRANTIES OF GENZYME

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

                                        4

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     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 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. Genzyme understands that the LLC
Interest has not 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 the LLC Interest cannot be
resold unless the LLC Interest is 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
LLC Interest be registered under the Securities Act or any state securities
laws.

     3.4 Experience. Genzyme has carefully reviewed the representations
concerning GTC and ATIII LLC contained in this Agreement and has had the
opportunity to make detailed inquiry concerning GTC, ATIII LLC and their
respective businesses and personnel. The officers of GTC 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 ATIII 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 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
ATIII 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.


                                        5

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                          ARTICLE 4 CONDITION PRECEDENT

     4.1 Clerk's Certificate. Contemporaneously with the execution of this
Agreement, GTC shall deliver to Genzyme resolutions of the Board of Directors of
GTC authorizing and approving all matters in connection with this Agreement and
the transactions contemplated hereby, certified by the Clerk of GTC as of the
date hereof.

                             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.

     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.


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     If to                  Genzyme Transgenics Corporation 
     GTC:                   Five Mountain Road Framingham,
                            Massachusetts 01701 
                            Attention: President 
                            Facsimile: (508) 370-3797

     with a copy to:        Genzyme Transgenics Corporation
                            Five Mountain Road
                            Framingham, Massachusetts 01701
                            Attention: General Counsel
                            Facsimile: (508) 370-3797.

     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 Officer
                            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 without regard to
any choice of law principle that would dictate the application of the laws of
another jurisdiction.

     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.

     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

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the same instrument.

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


                         GENZYME TRANSGENICS CORPORATION


                         By: /s/ John B. Green
                             ---------------------

                         Title: Vice President and Chief Financial Officer
                                ------------------------------------------

                         Date:  December 29, 1997
                                -----------------


                         GENZYME CORPORATION


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

                         Date:  December 29, 1998
                                -------------------

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