MEMBERSHIP INTEREST PURCHASE AGREEMENT



                                 by and between



                         PROGENICS PHARMACEUTICALS, INC.

                                       and

                               CYTOGEN CORPORATION



                              Dated April 20, 2006









<PAGE>

                                TABLE OF CONTENTS
<TABLE>
<CAPTION>

<S>                                                                                          <C>
                                                                                             Page
                                                                                             ----

ARTICLE I    DEFINITIONS AND DEFINED TERMS....................................................1

     Section 1.1  Definitions and Defined Terms...............................................1
                  -----------------------------

ARTICLE II   PURCHASE OF MEMBERSHIP INTEREST..................................................2

     Section 2.1  Purchase and Sale of Membership Interest....................................2
                  ----------------------------------------

     Section 2.2  Purchase Price..............................................................2
                  --------------

     Section 2.3  Allocation of Purchase Price................................................2
                  ----------------------------

     Section 2.4  Withholding Taxes...........................................................3
                  -----------------

ARTICLE III  CLOSING..........................................................................3

     Section 3.1  Closing.....................................................................3
                  -------

     Section 3.2  Deliveries at Closing.......................................................3
                  ---------------------

ARTICLE IV   REPRESENTATIONS AND WARRANTIES OF SELLER.........................................5

     Section 4.1  Ownership of Membership Interest; No Third Party Options....................5
                  --------------------------------------------------------

     Section 4.2  Corporate Organization and Good Standing....................................6
                  ----------------------------------------

     Section 4.3  Authorization and Effect of Agreement.......................................6
                  -------------------------------------

     Section 4.4  Consents and Approvals; No Violations.......................................6
                  -------------------------------------

     Section 4.5  Litigation..................................................................7
                  ----------

     Section 4.6  Performance of Obligations;  Other Matters..................................7
                  ------------------------------------------

     Section 4.7  Seller Sophistication; Excluded Information.................................7
                  -------------------------------------------

     Section 4.8  No Broker...................................................................8
                  ---------

     Section 4.9  No Misleading Statements....................................................8
                  ------------------------

ARTICLE V    REPRESENTATIONS AND WARRANTIES OF PURCHASER......................................8

     Section 5.1  Corporate Organization and Good Standing....................................8
                  ----------------------------------------

     Section 5.2  Authorization and Effect of Agreement.......................................9
                  -------------------------------------

     Section 5.3  Consents and Approvals; No Violations.......................................9
                  -------------------------------------

     Section 5.4  No Broker...................................................................9
                  ---------

     Section 5.5  Litigation..................................................................9
                  ----------

     Section 5.6  Performance of Obligations; Other Matters..................................10
                  -----------------------------------------

     Section 5.7  No Misleading Statements...................................................10
                  ------------------------


                                       i
<PAGE>


     Section 5.8  Third Parties..............................................................10
                  -------------

ARTICLE VI   COVENANTS.......................................................................10

     Section 6.1  Further Assurances.........................................................10
                  ------------------

     Section 6.2  Confidentiality............................................................11
                  ---------------

     Section 6.3  Non-Hiring.................................................................12
                  ----------

     Section 6.4  Termination of Rights and Liabilities Under PSMA Operating Agreement.......12
                  --------------------------------------------------------------------

     Section 6.5  Tax Matters................................................................13
                  -----------

ARTICLE VII  MISCELLANEOUS PROVISIONS........................................................14

     Section 7.1  Notices....................................................................14
                  -------

     Section 7.2  Expenses...................................................................15
                  --------

     Section 7.3  Successors and Assigns.....................................................15
                  ----------------------

     Section 7.4  Extension; Waiver..........................................................15
                  -----------------

     Section 7.5  Entire Agreement...........................................................16
                  ----------------

     Section 7.6  Amendments, Supplements, Etc...............................................16
                  ----------------------------

     Section 7.7  Applicable Law.............................................................16
                  --------------

     Section 7.8  Execution in Counterparts..................................................16
                  -------------------------

     Section 7.9  Titles and Headings........................................................16
                  -------------------

     Section 7.10 Invalid Provisions.........................................................16
                  ------------------

     Section 7.11 Publicity..................................................................16
                  ---------

     Section 7.12 Specific Performance.......................................................17
                  --------------------
</TABLE>


                                       ii
<PAGE>


Annex and Exhibits
------------------

Annex A - Definitions and Defined Terms

Exhibit A - Membership Assignment Agreement
Exhibit B - Amended and Restated PSMA/PSMP License
Exhibit C - Resignations
Exhibit D - Managing Representative Certificate
Exhibit E - Management Committee Waiver and Approval Certificate









                                       iii
<PAGE>


                     MEMBERSHIP INTEREST PURCHASE AGREEMENT

     This MEMBERSHIP  INTEREST PURCHASE AGREEMENT (this "Agreement") is made and
                                                         ---------
entered  into as of April 20,  2006 by and  between  Progenics  Pharmaceuticals,
Inc., a Delaware corporation  ("Purchaser") and Cytogen Corporation,  a Delaware
                                ---------
corporation ("Seller").
              ------

                                    RECITALS:

     WHEREAS,  Purchaser and Seller are members of PSMA Development Company LLC,
a Delaware limited  liability  company (the "Company"),  and have entered into a
                                             -------
Limited  Liability  Company  Agreement for the Company,  dated June 15, 1999, as
amended  by  Amendment  Number 1,  dated  March 22,  2002 (the  "PSMA  Operating
                                                                 ---------------
Agreement");
---------

     WHEREAS,  Purchaser  and Seller each own a 50%  membership  interest in the
Company;

     WHEREAS,  upon the terms and subject to the  conditions  set forth  herein,
Seller  desires to sell to  Purchaser,  and  Purchaser  desires to purchase from
Seller, Seller's 50% membership interest in the Company;

     WHEREAS,  Purchaser and Seller desire to terminate,  and/or acknowledge the
termination  of, any rights  granted to Seller  pursuant  to the PSMA  Operating
Agreement  which are personal in nature to Seller or otherwise not  transferable
or licensable, including, without limitation, the rights contained in Article IX
of the PSMA Operating Agreement; and

     WHEREAS,  following  the  Closing,  Purchaser,  as the sole  member  of the
Company,  intends to amend,  restate  and  supersede  in its  entirety  the PSMA
Operating  Agreement  by executing  an Amended and  Restated  Limited  Liability
Company Agreement for the Company.

     NOW, THEREFORE, in consideration of the mutual representations, warranties,
covenants and agreements herein contained, the parties hereto agree as follows:

                                   ARTICLE I

                          DEFINITIONS AND DEFINED TERMS

     Section 1.1   Definitions and Defined Terms.  Unless the context  otherwise
                   -----------------------------
requires  or as  otherwise  defined  herein,  capitalized  terms  used  in  this
Agreement shall have the meanings set forth in Annex A hereto.
                                               -------


                                       1
<PAGE>


                                   ARTICLE II

                         PURCHASE OF MEMBERSHIP INTEREST

     Section 2.1   Purchase and Sale of Membership Interest.  Upon the terms and
                   ----------------------------------------
subject  to the  conditions  set  forth  herein,  at and  as of the  Closing  as
described in Article III hereto, Seller shall sell, transfer, convey, assign and
             -----------
deliver to  Purchaser  all of  Seller's  membership  interests  in the  Company,
including all of Seller's economic interests in the capital, profits, losses and
distributions of the Company and all of Seller's other rights and interests as a
member  of the  Company,  including,  without  limitation,  voting  rights  (the
"Membership  Interest"),  and Purchaser  shall purchase and acquire from Seller,
 --------------------
all right, title and interest in the Membership Interest,  free and clear of any
Lien.

     Section 2.2   Purchase  Price.  In  consideration  of   the  conveyance  to
                   ---------------
Purchaser of the  Membership  Interest,  and subject to the terms and conditions
hereof, Purchaser shall pay to Seller an aggregate purchase price of $13,200,000
(the "Purchase Price"), payable by Purchaser on the Closing Date.
      --------------

     Section 2.3   Allocation of Purchase Price.
                   ----------------------------

     (a)  Purchaser and Seller shall allocate the sum of the Purchase  Price and
the  liabilities  of the  Company  among the assets of the Company in the manner
required by section  1060 of the Code and the Treasury  Regulations  thereunder.
Within  sixty  (60)  days  after  the  Closing  Date,  Purchaser  shall  use its
reasonable  efforts to prepare  and  deliver to Seller a proposed  IRS Form 8594
allocating  all such amounts as provided  herein,  and a statement  specifying a
methodology  for the  allocation of any  adjustments to the Purchase Price under
this  Agreement  (together,  the "Asset  Acquisition  Statement").  Seller shall
                                  -----------------------------
cooperate with Purchaser and Purchaser's  representatives in connection with the
preparation of such Asset  Acquisition  Statement,  including by furnishing such
information  and access to books,  records,  personnel and  properties as may be
reasonably   requested.   Both  Purchaser  and  Seller  shall  file  such  Asset
Acquisition  Statement  in the manner  required by Treasury  Regulation  section
1.1060-1(e). Such Asset Acquisition Statement shall become final and binding for
purposes  of this  Section  2.3  unless  Seller  objects in writing to the Asset
Acquisition  Statement within ten (10) days after Seller's  receipt thereof.  If
Seller so objects,  Purchaser  and Seller shall in good faith attempt to resolve
the dispute  within  sixty (60) days of written  notice to Purchaser of Seller's
objection.  Seller  and  Purchaser  agree to submit  any  unresolved  dispute to
arbitration  to  one  of  the  major   nationally-recognized   certified  public
accounting  firms (the  "Reviewing  Accountants"),  whose decision on the matter
                         ----------------------
shall be final and binding on the parties hereto. Purchaser and Seller will each
pay one-half of the fees and expenses of the Reviewing Accountant.  Seller shall
cooperate  with  Purchaser  and  Purchaser's  representatives,  as  well  as the
Reviewing  Accountant,  in  connection  with the  matters  contemplated  by this
Section 2.3,  including,  by furnishing  such  information  and access to books,
records, personnel and properties as may be reasonably requested.

     (b)  Each of Purchaser and Seller agrees to (i) prepare and timely file all
Tax  Returns in a manner  consistent  with the Asset  Acquisition  Statement  as
finalized


                                       2
<PAGE>


and  revised  in  accordance  with  Sections  2.3(a)  and 2.3(b) and (ii) act in
accordance with the Asset Acquisition  Statement for all Tax purposes, in either
case,  except  as  otherwise  required  by Law.  In the  event  that  any of the
allocations  determined  pursuant  to such  statement  are  disputed  by any Tax
Authority,  the Party receiving notice of such dispute shall promptly notify and
consult with the other Party hereto concerning the resolution of such dispute.

     Section 2.4   Withholding Taxes.  Purchaser shall be entitled to deduct and
                   -----------------
withhold from the amounts otherwise payable pursuant to this Agreement to Seller
such  amounts,  if any, as it is required to deduct and withhold with respect to
the making of such  payment  under any  provision  of federal,  state,  local or
foreign  Law  relating to Taxes.  To the extent that  amounts are so withheld by
Purchaser, Purchaser shall provide Seller with evidence of any amounts withheld,
and such withheld amounts shall  nonetheless be treated for all purposes of this
Agreement as having been paid to Seller.

                                  ARTICLE III

                                     CLOSING

     Section 3.1   Closing. Upon the terms and subject to the conditions of this
                   -------
Agreement,  the closing of the transactions  contemplated by this Agreement (the
"Closing")  will take place at the offices of Dewey  Ballantine LLP, 1301 Avenue
 -------
of the Americas, New York, New York on April 20, 2006 (the "Closing Date").
                                                            ------------

     Section 3.2   Deliveries at Closing. On the Closing Date:
                   ---------------------

     (a)  Seller  shall deliver,  or cause to be  delivered,  to  Purchaser  the
following:

          (i)    an Assignment of Membership Interest, duly executed  by Seller,
     in substantially  the form  attached hereto  as Exhibit A (the  "Membership
                                                     ---------        ----------
     Assignment");
     ----------

          (ii)   an amended and restated  PSMA/PSMP  License,  duly  executed by
     Seller,  in  substantially  the form  attached  hereto  as  Exhibit  B (the
                                                                 ----------
     "Amended and Restated PSMA/PSMP License");
      --------------------------------------

          (iii)  written resignations,  dated the Closing Date, duly executed by
     each Representative  appointed to the Management Committee and, if any, the
     Scientific  Advisory Board, by Seller,  in substantially the forms attached
     as Exhibit C (the "Resignations");
        ---------       ------------

          (iv)   a  certificate,  dated the  Closing Date,  notifying persons to
     whom certificates  contemplated by Section 4.1(h)(i) or Section 4.1(h)(iii)
     of  the  PSMA  Operating   Agreement  were  previously   delivered  of  the
     resignation of the Representatives and Managing Representative appointed by
     Seller, duly executed by the Managing Representative appointed by


                                       3
<PAGE>


     Seller,  in  substantially  the form  attached  hereto  as  Exhibit  D (the
                                                                 ----------
     "Managing Representative Certificate");
      -----------------------------------

          (v)    a  certificate, dated the Closing  Date, duly  executed by each
     Representative appointed by Seller certifying that the Management Committee
     has  (a) not  requested  any  additional  information,  representations  or
     opinions  pursuant to Section 6.2 of the PSMA Operating  Agreement and that
     the assignment of the Membership Interest to Purchaser is effective and (b)
     authorized and approved the execution, delivery and performance of, and the
     consummation of the transactions  contemplated by, the Amended and Restated
     PSMA/PSMP   License  Agreement  and  any  other  documents  or  instruments
     contemplated  thereby, in substantially the form attached hereto as Exhibit
                                                                         -------
     E (the "Management Committee Waiver and Approval Certificate");
     -       ----------------------------------------------------

          (vi)   a  certificate, dated the Closing  Date,  duly  executed by the
     Secretary or Assistant Secretary of Seller, on behalf of Seller, certifying
     as to (a) the attached copy of the resolutions of the Board of Directors of
     Seller  authorizing  and approving the execution,  delivery and performance
     of, and the consummation of the transactions contemplated by this Agreement
     and the  Collateral  Agreements  and any  other  documents  or  instruments
     contemplated  hereby or thereby,  and stating that the resolutions  thereby
     certified have not been amended,  modified,  revoked or rescinded,  and (b)
     the incumbency,  authority and specimen signature of each officer of Seller
     executing this Agreement,  the Collateral  Agreements or any other document
     or instrument contemplated hereby or thereby;

          (vii)  a  certificate  to  the  effect that,  as of the  Closing Date,
     Seller is not a foreign  person  within the meaning of section  1445 of the
     Code,  and the Treasury  Regulations  thereunder,  such  certificate  to be
     substantially  in  the  form  described  in  Treasury  Regulations  section
     1.1445-2(b)(2)(iv)(B); and

          (viii) such other duly executed  documents and  certificates as may be
     required to be delivered by Seller  pursuant to the terms of this Agreement
     or the Collateral Agreements or as may be reasonably requested by Purchaser
     prior to the Closing Date.

     (b)  Purchaser  shall  deliver,  or cause to be  delivered,  to Seller  the
following:

          (i)    the Purchase Price by wire transfer  of  immediately  available
     federal funds to an account  designated by Seller (which  designation shall
     be made by Seller not less than two Business Days before the Closing Date);

          (ii)   the Membership Assignment, duly executed by Purchaser;


                                       4
<PAGE>


          (iii)  Amended and Restated  PSMA/PSMP  License,  duly executed by the
     Company and Purchaser;

          (iv)   the Managing  Representatives  Certificate,  dated the  Closing
     Date, duly executed by the Managing Representative appointed by Purchaser;

          (v)    the Management Committee Waiver and Approval Certificate, dated
     the  Closing  Date,  duly  executed  by each  Representative  appointed  by
     Purchaser; and

          (vi)   a certificate, dated the  Closing  Date,  duly  executed by the
     Secretary,  Assistant Secretary or other officer of Purchaser, on behalf of
     Purchaser, certifying as to (a) the attached copy of the resolutions of the
     Board of Directors of Purchaser  authorizing  and approving the  execution,
     delivery  and  performance  of, and the  consummation  of the  transactions
     contemplated by, this Agreement and the Collateral Agreements and any other
     documents  or  instruments   contemplated  hereby,  and  stating  that  the
     resolutions thereby certified have not been amended,  modified,  revoked or
     rescinded, and (b) the incumbency, authority and specimen signature of each
     officer of Seller  executing this Agreement,  the Collateral  Agreements or
     any other document or instrument contemplated hereby or thereby.

                                   ARTICLE IV

                    REPRESENTATIONS AND WARRANTIES OF SELLER

     Except as  otherwise  disclosed  to  Purchaser  in a schedule  delivered to
Purchaser by Seller  prior to the  execution of this  Agreement  (with  specific
reference to the  representations and warranties in this Article IV to which the
                                                         ----------
information  in  such  schedule  relate)  (the  "Disclosure  Schedule"),  Seller
                                                 --------------------
represents and warrants to Purchaser as follows:

     Section 4.1   Ownership  of Membership  Interest;  No Third Party  Options.
                   ------------------------------------------------------------
Seller is the owner,  beneficially  and of record,  of the Membership  Interest,
free and clear of any Lien. The Membership  Interest  represents Seller's entire
ownership interest in the Company.  On the Closing Date, Seller will transfer to
Purchaser good and marketable  title to such Membership  Interest free and clear
of any Lien. The sale and delivery of the Membership Interest as contemplated by
this Agreement is not subject to any preemptive right, right of first refusal or
other restriction,  other than as set forth in PSMA Operating Agreement.  Except
for this  Agreement  and the PSMA  Operating  Agreement,  there are no  existing
agreements,  options,  commitments  or other rights  relating to the  Membership
Interest or granting any Person the right to acquire the Membership  Interest or
any interest therein.  The Membership  Interest represents 50% of the membership
interests in the Company and, to Seller's knowledge, no one other than Purchaser
owns or has any right or option to acquire  any  membership  or other  ownership
interest in the Company.


                                       5
<PAGE>


     Section  4.2  Corporate  Organization  and  Good  Standing.   Seller  is  a
                   --------------------------------------------
corporation duly  incorporated,  validly existing and in good standing under the
laws of the  State  of  Delaware  and  has all  requisite  corporate  power  and
authority to own,  lease,  operate and otherwise  hold its properties and assets
and to carry on its business as presently conducted. Seller is duly qualified or
licensed  to do  business as a foreign  corporation  and is in good  standing in
every  jurisdiction  in which the nature of the business  conducted by it or the
assets or properties owned or leased by it requires qualification,  except where
the  failure  to be so  qualified,  licensed  or in  good  standing  would  not,
individually  or in the  aggregate,  be  reasonably  likely  to have a  material
adverse effect on (i) its ability to consummate the transactions contemplated by
this Agreement or the Collateral  Agreements or (ii) the financial  condition of
Seller.

     Section 4.3   Authorization  and  Effect  of  Agreement.  Seller  has   all
                   -----------------------------------------
requisite  corporate  power and authority to execute and deliver this  Agreement
and the  Collateral  Agreements  and to perform its  obligations  hereunder  and
thereunder.  The  execution and delivery of this  Agreement  and the  Collateral
Agreements by Seller and the performance by Seller of its obligations  hereunder
and thereunder and the consummation by Seller of the  transactions  contemplated
hereby and thereby have been duly  authorized  by its Board of Directors  and no
other  corporate or other action on the part of Seller is necessary to authorize
the execution and delivery of this  Agreement and the  Collateral  Agreements or
the  consummation  of the  transactions  contemplated  hereby or  thereby.  This
Agreement and the Collateral  Agreements have been duly and validly executed and
delivered  by Seller and  constitute  legal,  valid and binding  obligations  of
Seller,  enforceable  against Seller in accordance with their terms,  subject to
applicable  bankruptcy,   insolvency,  fraudulent  conveyance,   reorganization,
moratorium and similar Laws affecting creditors' rights and remedies generally.

     Section 4.4   Consents and Approvals; No Violations. No filing with, and no
                   -------------------------------------
permit or Consent of any Governmental Authority or any other Person is necessary
for  the  consummation  by  Seller  of the  transactions  contemplated  by  this
Agreement or the  Collateral  Agreements.  Neither the execution and delivery of
this Agreement or the Collateral  Agreements by Seller nor the  consummation  by
Seller of the  transactions  contemplated  by this  Agreement or the  Collateral
Agreements nor compliance by Seller with any of the provisions hereof or thereof
will  (a)  conflict  with  or  result  in any  breach  of any  provision  of its
certificate of incorporation or by-laws, (b) result in a violation or breach of,
or  constitute  (with or without  due notice or lapse of time or both) a default
(or  give  rise to any  right  of  termination,  modification,  cancellation  or
acceleration or loss of material  benefits) under, any of the terms,  conditions
or  provisions  of any Contract to which Seller is a party or by which Seller or
any of its properties or assets may be bound, (c) result in any Lien on any part
of the Membership Interest,  (d) violate any permit or Law applicable to Seller,
or (e) violate the PSMA Operating  Agreement,  except in the case of clauses (b)
or (d), for violations, breaches or defaults which would not, individually or in
the aggregate, be reasonably likely to have a material adverse effect on (i) its
ability to consummate  the  transactions  contemplated  by this Agreement or the
Collateral Agreements or (ii) the Company.


                                       6
<PAGE>


     Section  4.5  Litigation.  There is no  action,  proceeding,  claim,  suit,
                   ----------
opposition,  challenge,  cancellation  proceeding,  reexamination,  interference
proceeding, charge or investigation (collectively, "Proceedings") pending or, to
                                                    -----------
Seller's  knowledge,  threatened,  that  relates to  Seller's  ownership  of the
Membership  Interest or that  questions  the  validity of this  Agreement or the
Collateral Agreements or any action taken or to be taken in connection with this
Agreement or the  Collateral  Agreements.  There are no  outstanding  judgments,
writs,  injunctions,  orders,  decrees or settlements that apply, in whole or in
part,  to  the  Membership  Interest  or  that  restrict  the  ownership  of the
Membership Interest in any way.

     Section 4.6   Performance of Obligations; Other Matters.
                   -----------------------------------------

     (a)  Other  than pursuant  to the  PSMA/PSMP  License,  the PSMA  Operating
Agreement  and/or  the  Services  Agreement,  neither  Seller  nor  any  of  its
Affiliates has ever been a party to any Contract with the Company and, except as
reflected in the Company's  audited  financial  statements  for the period ended
December 31, 2005,  the Company does not have, and has never had, any obligation
or liability,  contingent or otherwise, owing to Seller or any of its Affiliates
(including  pursuant to any  Proceeding  or order).  Immediately  following  the
Closing Date,  neither Seller nor any of its  Affiliates  will be a party to any
Contract with the Company other the Amended and Restated PSMA/PSMP License.

     (b)  No amounts are owing by the Company or Purchaser to Seller pursuant to
the PSMA Operating Agreement.

     (c)  Seller's Member's Percentage is equal to 50%. Seller is not in default
of any of its obligations under the PSMA Operating Agreement and has timely paid
in full  all  Capital  Contributions  required  to be made by it to the  Company
pursuant to the PSMA Operating  Agreement.  As of the date hereof no amounts are
owing by Seller to the Company or Purchaser.

     (d)  Seller has  reacquired  the  Manufacturing  Rights (as  defined in the
Amended and  Restated  PSMA/PSMP  License)  and granted a license  with  respect
thereto to the Company in accordance  with Section  2.2(e) of the PSMA Operating
Agreement.

     (e)  The Services Agreement  has  terminated  pursuant to Section  3(a)(ii)
thereof  following the discharge in full of Purchaser's  obligation  pursuant to
Section  2.2  of  the  PSMA  Operating  Agreement  to  make  additional  Capital
Contributions of up to $3 million to fund the Company's research and development
programs as budgeted in the work plans  described  in Section 2 of the  Services
Agreement.

     (f)  Immediately  after giving effect to the  transactions  contemplated by
this  Agreement and the  Collateral  Agreements,  neither  Seller nor any of its
Affiliates  will have an  ownership,  leasehold or other right or interest in or
to, or otherwise retain possession of, any Company Assets.

     Section 4.7   Seller Sophistication; Excluded Information.
                   -------------------------------------------


                                       7
<PAGE>


     (a)  Seller represents  that it (i) is a member of the  Company,  (ii) is a
sophisticated seller of the Membership Interest,  (iii) has adequate information
concerning  the  business  and  financial  condition  of the  Company to make an
informed  decision  regarding the sale of the  Membership  Interest and (iv) has
independently and without reliance upon Purchaser, and based on such information
as Seller has deemed  appropriate,  made its own  analysis and decision to enter
into this Agreement and the Collateral Agreements.

     (b)  Seller  acknowledges  that (i)  Purchaser  has not  given  Seller  any
investment  advice,  credit  information,  or opinion on whether the sale of the
Membership  Interest is prudent,  (ii)  Purchaser and Seller have differed as to
the development  priorities and budgets for the Company,  (iii) Seller has had a
full  opportunity to make inquiries of, and receive  satisfactory  answers from,
Purchaser regarding all aspects of the business, finances, intellectual property
and  prospects  of the  Company,  and (iv)  Seller has  determined,  taking into
account the  foregoing  acknowledgements  in this  Section  4.7(b),  to sell the
Membership Interest.

     Section  4.8  No Broker.  No agent, broker,  investment  banker,  financial
                   ---------
advisor  or other  firm or  Person is or will be  entitled  to any  broker's  or
finder's fee or any other  commission or similar fee in  connection  with any of
the transactions contemplated by this Agreement.

     Section 4.9   No Misleading  Statements.  No  representation or warranty by
                   -------------------------
Seller  in this  Agreement,  and no  statement  made  by  Seller  in the  Seller
Disclosure  Schedule (if any) or any certificate or other document  furnished to
Purchaser pursuant hereto, or in connection with the negotiation or execution of
this  Agreement,  contains any untrue  statement of a material  fact or omits to
state a material fact necessary to make any statement herein not misleading.

                                   ARTICLE V

                   REPRESENTATIONS AND WARRANTIES OF PURCHASER

     Purchaser represents and warrant to Seller as follows:

     Section  5.1  Corporate  Organization  and Good  Standing.  Purchaser  is a
                   -------------------------------------------
corporation duly  incorporated,  validly existing and in good standing under the
laws of the  State  of  Delaware  and  has all  requisite  corporate  power  and
authority to own,  lease,  operate and otherwise  hold its properties and assets
and to carry on its business as presently conducted. Purchaser is duly qualified
or licensed to do business as a foreign  corporation  and is in good standing in
every  jurisdiction  in which the nature of the business  conducted by it or the
assets or properties owned or leased by it requires qualification,  except where
the  failure  to be so  qualified,  licensed  or in  good  standing  would  not,
individually  or in the  aggregate,  be  reasonably  likely  to have a  material
adverse effect on its ability to consummate  the  transactions  contemplated  by
this Agreement or the Collateral Agreements.


                                       8
<PAGE>


     Section  5.2  Authorization  and  Effect of  Agreement.  Purchaser  has all
                   ----------------------------------------
requisite  corporate  power and authority to execute and deliver this  Agreement
and the  Collateral  Agreements  and to perform its  obligations  hereunder  and
thereunder.  The  execution and delivery of this  Agreement  and the  Collateral
Agreements  by Purchaser  and the  performance  by Purchaser of its  obligations
hereunder and thereunder and the  consummation by Purchaser of the  transactions
contemplated  hereby  and  thereby  have  been duly  authorized  by its Board of
Directors  and no other  corporate  or other  action on the part of Purchaser is
necessary to authorize  the  execution  and delivery of this  Agreement  and the
Collateral  Agreements  or the  consummation  of the  transactions  contemplated
hereby or thereby.  This Agreement and the Collateral  Agreements have been duly
and validly executed and delivered by Purchaser and constitute legal,  valid and
binding  obligations of Purchaser,  enforceable  against Purchaser in accordance
with their  terms,  subject to  applicable  bankruptcy,  insolvency,  fraudulent
conveyance,  reorganization,  moratorium and similar Laws  affecting  creditors'
rights and remedies generally.

     Section 5.3   Consents and Approvals; No Violations. No filing with, and no
                   -------------------------------------
permit or Consent of any Governmental Authority or any other Person is necessary
for the  consummation  by Purchaser  of the  transactions  contemplated  by this
Agreement or the  Collateral  Agreements.  Neither the execution and delivery of
this Agreement or the Collateral Agreements by Purchaser nor the consummation by
Purchaser of the  transactions  contemplated by this Agreement or the Collateral
Agreements  nor  compliance by Purchaser  with any of the  provisions  hereof or
thereof will (a) conflict  with or result in any breach of any  provision of its
certificate of incorporation or by-laws, (b) result in a violation or breach of,
or  constitute  (with or without  due notice or lapse of time or both) a default
(or  give  rise to any  right  of  termination,  modification,  cancellation  or
acceleration or loss of material  benefits) under, any of the terms,  conditions
or  provisions  of any  Contract  to  which  Purchaser  is a party  or by  which
Purchaser  or any of its  properties  or assets may be bound or (c)  violate any
permit or Law applicable to Purchaser,  except in the case of clauses (b) or (c)
for  violations,  breaches or defaults which would not,  individually  or in the
aggregate, be reasonably likely to have a material adverse effect on its ability
to consummate the transactions  contemplated by this Agreement or the Collateral
Agreements.

     Section  5.4  No Broker.  No agent, broker,  investment  banker,  financial
                   ---------
advisor  or other  firm or  Person is or will be  entitled  to any  broker's  or
finder's fee or any other  commission or similar fee in  connection  with any of
the transactions contemplated by this Agreement.

     Section 5.5   Litigation. There is no Proceeding pending or, to Purchaser's
                   ----------
knowledge,  threatened,  that  questions  the validity of this  Agreement or the
Collateral Agreements or any action taken or to be taken in connection with this
Agreement or the  Collateral  Agreements.  There are no  outstanding  judgments,
writs,  injunctions,  orders,  decrees or settlements that apply, in whole or in
part,  to  the  Membership  Interest  or  that  restrict  the  ownership  of the
Membership Interest in any way.


                                       9
<PAGE>


     Section 5.6   Performance of Obligations; Other Matters.
                   -----------------------------------------

     (a)  Other than  pursuant  to the  PSMA/PSMP  License,  the PSMA  Operating
Agreement  and/or  the  Services  Agreement,  neither  Purchaser  nor any of its
Affiliates  has been a party to any  Contract  with the Company  and,  except as
reflected in the Company's  audited  financial  statements  for the period ended
December 31, 2005,  the Company does not have,  and did not have, any obligation
or  liability,  contingent  or  otherwise,  owing  to  Purchaser  or  any of its
Affiliates (including pursuant to any legal proceeding or order).

     (b)  No amounts are owing by the Company or Seller to Purchaser pursuant to
the PSMA Operating Agreement.

     (c)  Purchaser's  Member's Percentage is equal to 50%.  Purchaser is not in
material  default of any of its obligations  under the PSMA Operating  Agreement
and has timely paid in full all Capital Contributions  required to be made by it
to the Company pursuant to the PSMA Operating  Agreement.  As of the date hereof
no amounts are owing by Purchaser to the Company or Seller.

     Section 5.7   No Misleading Statements.  No  representation  or warranty by
                   ------------------------
Purchaser in this Agreement, and no statement made by Purchaser in the Purchaser
Disclosure  Schedule (if any) or any certificate or other document  furnished to
Purchaser pursuant hereto, or in connection with the negotiation or execution of
this  Agreement,  contains any untrue  statement of a material  fact or omits to
state a material fact necessary to make any statement herein not misleading.

     Section 5.8   Third Parties.  Except for the entity  disclosed by Purchaser
                   -------------
to Seller in connection with the inquiries referred to in clause (iv) of Section
4.7(b),  Purchaser has had no discussions  with any third parties  regarding any
transactions  involving  the Company or its assets  reasonably  likely to have a
material effect on the Company or the economic value of the Membership Interest.

                                   ARTICLE VI

                                    COVENANTS

     Section 6.1   Further Assurances. From time to time after the Closing Date,
                   ------------------
without  additional  consideration,  each party hereto will (or, if appropriate,
cause its Affiliates to) execute and deliver such further  instruments  and take
such other action as may be necessary or reasonably requested by the other party
to make  effective  the  transactions  contemplated  by this  Agreement  and the
Collateral  Agreements and to provide the other party with the intended benefits
of this Agreement and the Collateral Agreements. Without limiting the foregoing,
upon reasonable  request of Purchaser,  Seller shall, and Seller shall cause its
Affiliates to, as  applicable,  execute,  acknowledge,  notarize and deliver all
such  further  assurances,  deeds,  assignments,  powers of  attorney  and other
instruments and paper as may be required to sell, transfer,  assign,  convey and
deliver  to  Purchaser  all  right,  title  and  interest  in,  to and under the
Membership Interest or


                                       10
<PAGE>


provide evidence of such sale,  transfer,  assignment,  conveyance and delivery.
If,  notwithstanding  Section 4.6(f) and any remedies available to Purchaser for
breach thereof, the Seller or any of its Affiliates shall, following the Closing
Date,  have an  ownership,  leasehold  or other  right or  interest in or to, or
otherwise  retain  possession of, any Company Assets,  Seller shall,  and Seller
shall  cause its  Affiliates  to,  without  additional  consideration,  promptly
transfer, assign, convey and deliver such Company Assets to the Company. Without
limiting the  foregoing,  upon  reasonable  request of Purchaser or the Company,
Seller  shall,  and Seller shall cause its  Affiliates  to,  without  additional
consideration,  execute,  acknowledge,  notarize  and deliver  all such  further
assurances,  deeds,  assignments,  powers of attorney and other  instruments and
paper as may be required to provide  verification,  confirmation  or evidence of
the Company's rights, titles and interests in, to and under the Company Assets.

     Section 6.2   Confidentiality.
                   ---------------

     (a)  As used herein, "Confidential  Information" means all confidential and
                           -------------------------
proprietary  business,  technical,  or financial  information  received from, or
relating to, Purchaser or the Company.

     (b)  In order to  protect the  Confidential Information  that was developed
by, or has become available to Seller, Seller agrees as follows:

          (i)    It will  make no use of any Confidential Information  except in
     furtherance  of  the  purposes   contemplated  by  this  Agreement  or  the
     Collateral Agreements.

          (ii)   From the Closing Date and for a period of five  years (5) years
     thereafter,  it will not,  without the prior written  consent of Purchaser,
     disclose to any third party  Confidential  Information (which shall include
     the terms or existence of this Agreement,  the Collateral  Agreements,  the
     PSMA Operating Agreement or the PSMA/PSMP License or other matters relating
     to the transactions contemplated hereby and thereby).

          (iii)  Notwithstanding the foregoing:

                 Seller  may disclose  Confidential Information  to those of its
                 representatives,  employees  and  agents ("Agents")  who have a
                                                            ------
                 need  to  know such  Confidential  Information  in  relation to
                 the  matters discussed herein and who are under  obligations of
                 confidentiality  and non-use  consistent  with those set  forth
                 herein. Any unauthorized disclosure of Confidential Information
                 by Seller's  Agents shall be a breach by Seller of this Section
                 6.2.

                 Disclosure  of  Confidential  Information is  permitted to  the
                 extent that such  disclosure is required pursuant to applicable
                 Laws,  provided,  however,  that Seller  shall promptly  notify
                        --------   -------
                 Purchaser in writing of the existence or


                                       11
<PAGE>

                 imposition of any such requirement or order and cooperate  with
                 Purchaser  in seeking an  appropriate protective order or other
                 reliable assurance that confidential treatment will be accorded
                 the Confidential Information.

     (c)  The provisions governing confidentiality and non-use contained in this
Section 6.2 shall not apply to any Confidential Information which:

          (i)    was in the public  domain or the subject of public knowledge at
     the time of disclosure;

          (ii)   becomes part of the  public  domain  or the  subject  of public
     knowledge through no breach by or act of default of Seller; or

          (iii)  is obtained by Seller from a thirdv  party other than in breach
     of a legal or  contractual obligation of confidentiality owed by such third
     party to Purchaser  (or the Company) in respect  thereof,  the existence of
     which such obligation was known or should have been known by Seller.

     (d)  Any Confidential  Information  of the  Company  conveyed  by Seller to
Purchaser or the Company  prior to the Closing Date for the exclusive use of the
Company  shall  remain  subject to the  confidentiality  and non-use  provisions
contained in this Section 6.2.

     Section 6.3   Non-Hiring. From the Closing Date and for a period of two (2)
                   ----------
years thereafter,  Seller shall not, and shall cause its Affiliates not to, hire
as an employee or engage as a consultant any person who is employed by or who is
a consultant to or is otherwise  affiliated  with the Company,  or to solicit or
induce  any such  person to  terminate  his or her  employment,  consultancy  or
affiliation  with the  Company.  In  addition,  from the Closing  Date and for a
period of two (2) years  thereafter,  neither  Purchaser nor Seller  shall,  and
Purchaser and Seller shall cause their respective  Affiliates not to, hire as an
employee  or engage as a  consultant  any person who is  employed by or who is a
consultant  to or otherwise  affiliated  with the other party,  or to solicit or
induce  any such  person to  terminate  his or her  employment,  consultancy  or
affiliation  with the  other  party.

     Section 6.4   Termination  of Rights and Liabilities  Under PSMA  Operating
                   -------------------------------------------------------------
Agreement.
---------

     (a)  From and after the Closing,  Seller  shall cease to be a member of the
Company and shall have no further powers,  rights,  or privileges as a Member or
otherwise under the PSMA Operating Agreement.

     (b)  Each of Seller  and  Purchaser  hereby  agree  and  acknowledge  that,
immediately upon consummation of the Closing,  all rights of Seller contained in
the PSMA Operating Agreement which are not transferable to Purchaser pursuant to
this agreement,  including,  without limitation, the rights contained in Article
IX of the PSMA


                                       12
<PAGE>


Operating  Agreement,  shall be  terminated as of the Closing and shall be of no
further force and effect.

     (c)  Notwithstanding  Section 11.9 of the PSMA  Operating  Agreement or any
other provision of the PSMA Operating Agreement to the contrary, effective as of
the Closing,  each of Purchaser and Seller hereby  releases and  discharges  the
other,  and  Seller  hereby  releases  and  discharges  the  Company,  from  any
liability,  claim, demand, debt, obligation,  dispute, action or cause of action
("Claims")  that Seller or Purchaser may have against each other, or that Seller
  ------
may have  against the Company,  pursuant to the PSMA  Operating  Agreement,  the
Delaware Limited  Liability Company Act or otherwise by reason of being a member
of the Company,  including  all Claims  which shall have already  accrued at the
time of the Closing and all Claims which may accrue after the Closing; provided,
                                                                       --------
however,  that nothing in this Section  6.4(c) shall  operate as (i) a waiver by
-------
either party hereto of the terms,  conditions,  representations  and  warranties
contained in this Agreement,  the Amended and Restated  PSMA/PSMP  License,  the
Collateral  Agreements or any other agreement  contemplated herein or therein or
(ii) a release or  discharge  of any Claims  which  Purchaser or Seller may have
against the other with  respect to any such terms,  conditions,  representations
and warranties.

     (d)  From and after the Closing, Seller's  consent shall not be required to
effect  any  amendment,   restatement  or  termination  of  the  PSMA  Operating
Agreement.

     Section 6.5   Tax Matters.  With  respect  to the  Taxable  Periods  of the
                   -----------
Company  ending on or before the Closing  Date,  Seller and  Purchaser  agree as
follows:

     (a)  The tax matters  partner ("Tax  Matters  Partner")  of the Company for
                                     ---------------------
purposes  of  Section  6223 of the Code  shall be  Purchaser.  Unless  otherwise
required by law, the Tax Matters  Partner shall not take any action  pursuant to
this Section 6.5 unless such action has been consented to by Seller.

     (b)  All elections by the Company for income and franchise tax purposes and
all determinations regarding the book basis, depreciation or amortization of any
Company  assets,  and all other matters  relating to all tax returns  (including
amended returns),  including the  characterization  and allocation of income and
loss,  filed by the  Company,  including  tax audits  and  related  matters  and
controversies,  shall be made and  conducted  by the Tax Matters  Partner at the
expense of the  Company,  subject to the  approval  of Seller.  The Tax  Matters
Partner  shall,  at the expense of the  Company  and subject to the  approval of
Seller,  cause to be  prepared  and filed  all tax  returns  (including  amended
returns)  required to be filed by the Company;  provided,  however,  that Seller
                                                --------   -------
shall have the  opportunity to review any and all tax returns in advance of such
filing.  In the event of a dispute  between Seller and Purchaser  concerning the
preparation and filing of the Company's tax returns,  Seller and Purchaser agree
to submit the dispute to arbitration  to one of the major  nationally-recognized
certified public accounting  firms,  whose decision on the matter shall be final
and binding.


                                       13
<PAGE>


     (c)  The Tax Matters Partner shall be responsible  for all  negotiations on
behalf of the Company with the IRS or the Departments of the Treasury or Justice
or any state or local tax authority  with respect to the income tax treatment of
Company items, and shall provide Seller with the opportunity,  at the expense of
the Company,  to participate in any such  negotiations.  The Tax Matters Partner
shall not bind  Seller to a  settlement  agreement  unless  Seller has given its
written consent to such agreement.

                                  ARTICLE VII

                            MISCELLANEOUS PROVISIONS

     Section 7.1   Notices.  All notices  and other  communications  required or
                   -------
permitted  hereunder will be in writing and, unless  otherwise  provided in this
Agreement,  will be deemed to have been duly given when  delivered  in person or
when dispatched by electronic  facsimile transfer  (confirmed in writing by mail
simultaneously  dispatched) or one (1) Business Day after having been dispatched
by a nationally recognized overnight courier service to the appropriate party at
the address specified below:

     (a)  If to Purchaser, to:

                 Progenics Pharmaceuticals, Inc.
                 777 Old Saw Mill River Road
                 Tarrytown, New York 10591
                 Telecopy: 914-789-2856
                 Attention:  Mark R. Baker, Senior Vice President
                             & General Counsel

                 with a copy (which shall not constitute notice) to:

                 Dewey Ballantine LLP
                 1301 Avenue of the Americas
                 New York, New York 10019
                 Telecopy: 212-259-6333
                 Attention:  Donald Murray


                                       14
<PAGE>


     (b)  If to Seller, to:

                 Cytogen Corporation
                 650 College Road East
                 Princeton, New Jersey 08540
                 Telecopy: 609-452-2317
                 Attention:   William J. Thomas, Senior Vice President
                              and General Counsel

                 with a copy (which shall not constitute notice) to:

                 Morgan, Lewis & Bockius LLP
                 502 Carnegie Center
                 Princeton, New Jersey 08540
                 Telecopy: 609-919-6701
                 Attention: Randall B. Sunberg

or to such other  address or  addresses  as any such party may from time to time
designate as to itself by like notice.

     Section 7.2   Expenses. Except as otherwise expressly provided herein, each
                   --------
party hereto will pay any expenses  incurred by it incident to this Agreement or
the Collateral  Agreements and in preparing to consummate and  consummating  the
transactions provided for herein and therein.

     Section 7.3   Successors and Assigns. No party to this Agreement may assign
                   ----------------------
any of its rights under this Agreement  without the prior written consent of the
other parties hereto;  provided,  however,  that Purchaser may assign its rights
                       --------   -------
and  obligations  under this  Agreement,  without the prior  written  consent of
Seller, to an Affiliate of Purchaser.  Subject to the preceding  sentence,  this
Agreement  will  apply to, be  binding in all  respects  upon,  and inure to the
benefit of the successors and permitted  assigns of the parties hereto.  Nothing
expressed or referred to in this  Agreement will be construed to give any Person
other than the parties to this Agreement any legal or equitable right, remedy or
claim  under  or  with  respect  to  this  Agreement  or any  provision  of this
Agreement.  This  Agreement and all of its provisions and conditions are for the
sole and exclusive benefit of the parties to this Agreement and their successors
and permitted assigns.

     Section 7.4   Extension; Waiver. Any party hereto may, by written notice to
                   -----------------
the other  parties  hereto  (a) extend  the time for  performance  of any of the
obligations of the other party under this Agreement,  (b) waive any inaccuracies
in the  representations  or  warranties  of the other  party  contained  in this
Agreement,  (c) waive  compliance with any of the conditions or covenants of the
other party  contained in this  Agreement or (d) waive or modify  performance of
any of the obligations of the other party under this Agreement; provided that no
                                                                --------
such party hereto may,  without the prior  written  consent of the other parties
hereto, make or grant such extension of time, waiver


                                       15
<PAGE>


of  inaccuracies  or compliance or waiver or  modification  of performance  with
respect to its representations,  warranties,  conditions or covenants hereunder.
Except as  provided  in the  immediately  preceding  sentence,  no action  taken
pursuant to this  Agreement  will be deemed to constitute a waiver of compliance
with any representations,  warranties, conditions or covenants contained in this
Agreement  and will not operate or be  construed  as a waiver of any  subsequent
breach, whether of a similar or dissimilar nature.

     Section 7.5   Entire  Agreement.   This  Agreement,   which  includes   the
                   -----------------
schedules and exhibits hereto,  supersedes any other agreement,  whether written
or oral,  that may have been made or entered  into by any party  relating to the
matters  contemplated by this Agreement and constitutes the entire  agreement by
and among the parties hereto.

     Section 7.6   Amendments,  Supplements, Etc.  This Agreement may be amended
                   -----------------------------
or supplemented at any time by additional  written agreements as may mutually be
determined by Purchaser  and Seller to be  necessary,  desirable or expedient to
further  the  purposes of this  Agreement  or to clarify  the  intention  of the
parties hereto.

     Section  7.7  Applicable  Law.  This  Agreement  shall be  governed  by and
                   ---------------
construed under the laws of the State of Delaware  applicable to agreements made
and to be performed in Delaware.

     Section 7.8   Execution in Counterparts.  This Agreement may be executed in
                   -------------------------
two (2) or more counterparts,  each of which will be deemed an original, but all
of which  together will  constitute  one (1) and the same  agreement.  Facsimile
and/or electronically  delivered signatures to this Agreement and all Collateral
Agreements  executed in connection  herewith  shall be binding on the parties to
this Agreement.

     Section 7.9   Titles and Headings.  Titles and headings  to sections herein
                   -------------------
are inserted for  convenience  of reference  only,  and are not intended to be a
part of or to affect the meaning or interpretation of this Agreement.

     Section 7.10  Invalid Provisions.  If any  provision of  this Agreement  is
                   ------------------
held to be illegal,  invalid or  unenforceable  under any present or future Law,
and if the rights or obligations  under this Agreement of Seller on the one hand
and  Purchaser on the other hand will not be materially  and adversely  affected
thereby, (a) such provision will be fully severable,  (b) this Agreement will be
construed and enforced as if such illegal,  invalid, or unenforceable  provision
had  never  comprised  a part  hereof,  (c)  the  remaining  provisions  of this
Agreement  will  remain in full force and effect and will not be affected by the
illegal,  invalid,  or  unenforceable  provision or by its  severance  from this
Agreement and (d) in lieu of such illegal,  invalid or unenforceable  provision,
there will be added automatically as a part of this Agreement a legal, valid and
enforceable  provision  as  similar  in  terms  to  such  illegal,   invalid  or
unenforceable provision as may be possible.

     Section 7.11  Publicity.  Except as otherwise required by applicable Law or
                   ---------
the rules and regulations of any national securities  exchange,  no party hereto
shall


                                       16
<PAGE>


issue any press release or otherwise  make any public  statement with respect to
the  transactions  contemplated  by this Agreement or the Collateral  Agreements
without prior  consultation with and consent of the other parties hereto,  which
consent shall not be unreasonably withheld, conditioned or delayed.

     Section 7.12  Specific Performance. The parties hereto agree that if any of
                   --------------------
the provisions of this Agreement or the Collateral Agreements were not performed
in accordance with their specific terms or were otherwise breached,  irreparable
damage would occur,  no adequate  remedy at law would exist and damages would be
difficult  to  determine,  and that the  parties  hereto  shall be  entitled  to
specific  performance  of the terms hereof or thereof,  in addition to any other
remedy at law or equity.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]






                                       17
<PAGE>


     IN WITNESS  WHEREOF,  the parties hereto have executed this Agreement as of
the day and year first above written.



                              PROGENICS PHARMACEUTICALS, INC.



                              By: /s/ Mark R. Baker
                                 -----------------------------------------------
                                 Name:   Mark R. Baker
                                 Title:  Senior Vice President, General Counsel
                                         & Secretary




                              CYTOGEN CORPORATION



                              By: /s/ Michael D. Becker
                                 -----------------------------------------------
                                 Name: Michael D. Becker
                                 Title: President and Chief Executive Officer








            Signature Page to Membership Interest Purchase Agreement


<PAGE>


                                     Annex A
                                     -------

                          DEFINITIONS AND DEFINED TERMS

     (a)  As  used  in  this  Agreement,  the  following  terms  shall  have the
following meanings:

          "Affiliate"  shall mean with  respect to any Person,  any other Person
           ---------
who,  directly or  indirectly,  controls,  is  controlled  by or is under common
control with that Person. For purposes of this definition,  a Person has control
of another Person if it has the direct or indirect ability or power to direct or
cause the  direction  of  management  policies of such other Person or otherwise
direct the affairs of such other Person,  whether through  ownership of at least
fifty percent (50%) of the voting  securities of such other Person,  by Contract
or otherwise.  The term Affiliate shall not include the Company as it relates to
the  Purchaser or Seller,  as the case may be,  through and  including  the date
hereof.

          "Business Day" shall mean a day other than a Saturday, Sunday or other
           ------------
day on which  commercial  banks in New York are authorized or required by Law to
close.

          "Code" shall mean the Internal Revenue Code of 1986, as amended.
           ----

          "Collateral  Agreements" shall mean the Amended and Restated PSMA/PSMP
           ----------------------
License and the Membership Assignment.

          "Company   Assets"  shall  mean  any  properties  or  assets  (whether
           ----------------
tangible,  intangible, real, personal or mixed), which are used in, held for use
in,  necessary  for or  otherwise  relate to or arise from the  business  of the
Company, as conducted on the Closing Date (other than the rights licensed to the
Company pursuant to the Amended and Restated PSMA/PSMP License Agreement).

          "Consent" shall mean any consent, approval or authorization of, notice
           -------
to, or designation, registration, declaration or filing with, any Person.

          "Contract" shall mean any note, bond, mortgage,  indenture,  contract,
           --------
agreement,  permit, license, lease, purchase order, sales order,  arrangement or
other  commitment,  obligation  or  understanding,  written or oral,  to which a
Person is a party or by which a Person or its assets or properties are bound.

          "Governmental  Authority"  shall  mean any  federal,  state,  local or
           -----------------------
foreign  government  or any  subdivision,  agency,  instrumentality,  authority,
department,  commission, board or bureau thereof or any federal, state, local or
foreign court, tribunal or arbitrator.

          "knowledge"  shall mean the  knowledge any Person would have after due
           ---------
inquiry.

          "IRS" means the United States Internal Revenue Service.
           ---


                                       A1
<PAGE>


          "Laws" shall mean all federal,  state,  local or foreign laws, orders,
           ----
writs,  injunctions,   decrees,  ordinances,  awards,  stipulations,   statutes,
judicial or administrative doctrines, rules or regulations enacted, promulgated,
issued or entered by a Governmental Authority.

          "Liens" shall mean,  other than Permitted  Liens, all title defects or
           -----
objections,  mortgages, liens, claims, charges, pledges or other encumbrances of
any nature whatsoever,  including, without limitation, licenses, leases, chattel
or  other  mortgages,   collateral   security   arrangements,   pledges,   title
imperfections,  defect or objection liens,  security interests,  conditional and
installment sales agreements,  easements,  encroachments or restrictions, of any
kind and  other  title  or  interest  retention  arrangements,  reservations  or
limitations of any nature.

          "Management Committee" shall have the meaning ascribed to that term in
           --------------------
the PSMA Operating Agreement.

          "Managing Representative" shall have the meaning ascribed to that term
           -----------------------
in the PSMA Operating Agreement.

          "Member"  shall  have the  meaning  ascribed  to that term in the PSMA
           ------
Operating Agreement.

          "Member's  Percentage" shall have the meaning ascribed to that term in
           --------------------
the PSMA Operating Agreement.

          "Permitted Liens" mean (a) Liens for current real or personal property
           ---------------
Taxes not yet due and  payable  and with  respect to which the Seller  maintains
adequate  reserves,  (b) workers',  carriers' and mechanics' or other like Liens
incurred in the ordinary course of business with respect to which payment is not
due and that do not impair the  conduct of  business  or the present or proposed
use of the affected  property and (c) Liens that are  immaterial  in  character,
amount, and extent and which do not detract from the value or interfere with the
present or proposed use of the properties they affect.

          "Person"  shall  mean  any  individual,  partnership,  joint  venture,
           ------
corporation,  limited liability  company,  trust,  unincorporated  organization,
Governmental Authority or other entity.

          "PSMA/PSMP  License" means the PSMA/ PSMP License Agreement dated June
           ------------------
15, 1999 by and among Purchaser, Seller and the Company.

          "Representative"  shall have the meaning  ascribed to that term in the
           --------------
PSMA Operating Agreement.

          "Scientific  Advisory  Board" shall have the meaning  ascribed to that
           ---------------------------
term in the PSMA Operating Agreement.

          "Services  Agreement" means the Services Agreement dated June 15, 1999
           -------------------
by and among Purchaser, Seller and the Company.


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<PAGE>


          "Tax" or  "Taxes"  means  any  taxes and  similar  government  charges
           ---       -----
(including taxes on or with respect to net or gross income, franchise,  profits,
gross  receipts,  capital,  sales,  use,  ad  valorem,  value  added,  transfer,
registration,  capital stock,  license,  payroll,  employment,  social security,
unemployment,   severance,  real  or  personal  property,  excise,  recordation,
estimated  taxes,  withholding and stamp),  together with any interest  thereon,
penalties,  fines  and  additions  to tax with  respect  thereto,  imposed  by a
Governmental Authority.

          "Tax  Authority"   shall  mean  any   Governmental   Authority  having
           --------------
jurisdiction over the assessment, determination, collection or imposition of any
Tax.

          "Tax Returns" shall mean any report, return, election,  declaration or
           -----------
other  filing  required  to be  filed  with  any Tax  Authority,  including  any
amendments thereto.

          "Taxable  Period" shall mean any taxable year or any other period that
           ---------------
is treated as a taxable year, with respect to which any Tax may be imposed under
any applicable statute, rule, or regulation.

          "Treasury Regulations" shall mean the regulations, including temporary
           --------------------
regulations,  promulgated  under the Code, as the same may be amended  hereafter
from  time  to  time   (including   corresponding   provisions   of   succeeding
regulations).


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<PAGE>


     (b)  Each of the  following  terms is  defined  in the  Section  set  forth
opposite such term:

     Term                                                            Section
     ----                                                            -------
     Agents......................................................    6.2(b)(iii)
     Agreement...................................................    Preamble
     Amended and Restated PSMA/PSMP License......................    3.2(a)(ii)
     Asset Acquisition Statement.................................    2.3(a)
     Claims......................................................    6.4(c)
     Closing.....................................................    3.1
     Closing Date................................................    3.1
     Company.....................................................    Recitals
     Confidential Information....................................    6.2(a)
     Disclosure Schedule.........................................    Article IV
     Management Committee Waiver and Approval Certificate........    3.2(a)(v)
     Managing Representative Certificate.........................    3.2(a)(iv)
     Membership Assignment.......................................    3.2(a)(i)
     Membership Interest.........................................    2.1
     Proceedings.................................................    4.5
     PSMA Operating Agreement....................................    Recitals
     Purchase Price..............................................    2.2
     Purchaser...................................................    Preamble
     Resignations................................................    3.2(a)(iii)
     Reviewing Accountants.......................................    2.3(a)
     Seller......................................................    Preamble
     Tax Matters Partner.........................................    6.5(a)





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