EXHIBIT 10.1

                              AMENDMENT NUMBER 1 TO

                     LIMITED LIABILITY COMPANY AGREEMENT OF
                          PSMA DEVELOPMENT COMPANY LLC


         This  Amendment  Number 1 is made this  22nd day of March,  2002 by and
between Progenics  Pharmaceuticals,  Inc., a Delaware corporation ("Progenics"),
CYTOGEN  Corporation,  a Delaware  corporation  ("CYTOGEN") and PSMA Development
Company LLC, a Delaware Limited  Liability  company (the "Company"),  and amends
the Limited Liability Company Agreement,  dated June 15, 1999 (the "Agreement"),
among  Progenics,  CYTOGEN and the Company.  Unless  otherwise  defined  herein,
capitalized  terms used in this  Amendment  Number 1 shall  have the  respective
meanings ascribed to such terms in the Agreement.

         WHEREAS, the  parties to the  Agreement  desire to amend,  clarify and
supplement the provisions of the Agreement;

         NOW,  THEREFORE,  Progenics,  CYTOGEN and the Company  hereby  agree as
follows:

1.       Section 2.2(c)(i)of the Agreement is hereby amended to read as follows:

                  (c) (i) The  Management  Committee  may from time to time,  in
         connection  with  preparing  the Budget  (as  defined  hereinafter)  or
         otherwise,   call  for  the   Members   to  make   additional   capital
         contributions (the "Additional Capital Contributions"),  in which event
         the Management  Committee  shall give notice to each Member of: (A) the
         total amount of the Additional  Capital  Contribution being called; (B)
         the reason the Additional  Capital  Contribution  is being called;  (C)
         each  Member's  proportionate  share of the  total  Additional  Capital
         Contribution  (determined in accordance with this Section  2.2(c);  and
         (D) the date the Additional  Capital  Contribution  is due and payable,
         which date shall not,  without the written  consent of the Members,  be
         less than 30 nor more than 90  calendar  days after the notice has been
         given. A Member's share of the total  Additional  Capital  Contribution
         shall be equal to the product  obtained  by  multiplying  the  Member's
         Percentage and the total Additional Capital  Contribution  required.  A
         Member's  share shall be payable in cash,  by  certified  check or wire
         transfer.  No Additional Capital Contribution by any Member may be made
         or required to be made on an in-kind or any other non-cash basis unless
         consented  to in writing by each of the  Members.  Upon  payment of the
         Additional  Capital  Contributions,  the Capital  Contributions of each
         Member shall be  adjusted.  Without the prior  written  consent of each
         Member,  which consent may be withheld,  with or without reason, in the
         sole and absolute  discretion of each Member, the Management  Committee
         may not call for  additional  capital  contributions  in  excess  of an
         amount necessary to fund a Budget previously approved by the Management
         Committee, plus a reasonable reserve.

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2.       Section 2.4(a) of the Agreement is hereby amended to read as follows:

                  2.4.  Budget.  (a) The  Management  Committee  shall  promptly
         prepare a budget for the Company's activities covering such time period
         as the  Management  Committee  shall deem  appropriate.  The Budget may
         include provisions for Additional Capital  Contributions to made by the
         Members.  The Management  Committee shall meet to review and update the
         Budget on a semi-annual  basis or as it may  otherwise  determine to be
         appropriate.  Without the prior written  consent of each Member,  which
         consent  may be  withheld,  with or  without  reason,  in the  sole and
         absolute discretion of each Member, the Management  Committee shall not
         adopt for any 12-month period a Budget requiring funding by the Members
         in an amount in the  aggregate in excess of twice the funding  required
         by the  Members in the Budget  relating  to the  immediately  preceding
         12-month period.

3.       Section 4.1(g)(i)of the Agreement is hereby amended to read as follows:

                  (g)  (i)  If  submission  of  a  Deadlock  to  the  procedures
         described in Section 4.1(f) does not, within the time periods specified
         in  Section  4.1(f)  hereof,  result  in  resolution  of the  Deadlock,
         submission of the Deadlock to  arbitration  or agreement by the Members
         on an alternative dispute resolution procedure,  either party may elect
         to exercise the buy/sell  right  contained in this Section  4.1(g) (the
         "Buy/Sell  Right").  The Member electing to exercise the Buy/Sell Right
         (the  "Offeror")  shall  furnish in writing  to the other  Member  (the
         "Offeree") the Offeror's irrevocable, unconditional (except as provided
         herein) and binding offer (such notice being  referred to herein as the
         "Exercise Notice"),  at the Offeree's option, to purchase the Offeree's
         Interest or to sell the Offeror's  Interest for a cash  purchase  price
         determined in accordance with Section  4.1(g)(ii) hereof (the "Purchase
         Price").  The Exercise  Notice  shall set forth an amount  expressed in
         dollars and without contingencies (the "Valuation"), which amount shall
         be used to calculate, in accordance with Section 4.1(g)(ii) hereof, the
         Purchase Price.  The Valuation is intended to represent the amount that
         would be payable for 100% of the  Interests of the  Company.  Within 15
         calendar days after the Exercise Notice is given,  the Offeree may give
         notice to the  Offeror  of its  irrevocable,  unconditional  (except as
         provided herein) and binding election either:

               (1) to purchase the entire  Interest of the Offeror for an amount
          in cash equal to the Purchase Price; or

               (2) to sell its entire  Interest  to the Offeror for an amount in
          cash equal to the Purchase Price.

         Failure  of the  Offeree to give  notice of its  decision  within  such
         applicable  time period shall  constitute a conclusive  election by the
         Offeree to sell its entire Interest pursuant to this Section 4.1(g)(i).

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4.       Section 6.1 of the Agreement is hereby amended to read as follows:

                  6.1 Transfer or Withdrawal Prohibited;  Change of Control of a
         Member. (a) Except as expressly provided herein, no Member may withdraw
         from  the  Company  or sell,  assign,  transfer,  pledge,  hypothecate,
         mortgage  or  create  a  security  interest,  directly  or  indirectly,
         including by a grant of rights with respect to voting, distributions or
         other economic rights or management ("Transfer"), in all or any portion
         of its Interest  without the prior written  consent of the other Member
         (the "Consent-Required  Member"), and such Consent Required Member may,
         in the exercise of its reasoned business judgment,  withhold consent in
         its sole and absolute discretion.  Any purported withdrawal or Transfer
         of an Interest by a Member  without  the  requisite  consent in writing
         shall be null and void, and the Company and the Consent-Required Member
         shall be entitled to damages as a result of, and/or  injunctive  relief
         with respect to, any attempts to withdraw or Transfer.

                  (b) In the event that the Consent-Required  Member consents to
         a Transfer of a Member's Interest, such consent shall, unless expressly
         stated  otherwise,  be deemed a consent only to the  assignment of such
         Member's  economic  interest in profits,  losses and  distributions and
         shall not be deemed a consent to the  admission  of such  assignee as a
         member  of the  Company  as a  substitute  for the  assignor,  and such
         assignee  shall not have any of the rights of a member of the  Company,
         including,  without limitation,  voting rights, unless otherwise stated
         in writing by the  Consent-Required  Member.  Except to the extent that
         the  Consent-Required  Member has  consented  to the  admission  of the
         assignee as a member in the Company and/or consented to the exercise of
         voting  rights by the assignee,  the assigning  Member shall retain all
         voting rights in connection with the transferred Interest.

                  (c) In the  event  that at any time  there  occurs a Change of
         Control (as hereinafter  defined) of a Member,  such Member's  Interest
         shall  automatically  convert  into an  economic  interest  only in the
         profits, losses and distributions of the Company, and such Member shall
         not have  any  other  rights  of a Member  of the  Company,  including,
         without  limitation,  voting and marketing rights unless such Change of
         Control is a Qualifying  Corporate  Transaction (as defined below). For
         purposes  hereof,  a  "Change  in  Control"  shall  be  deemed  to have
         occurred:  (i) on the sale or other disposition of all or substantially
         all of the Member's  assets to any entity,  person or related  group of
         persons;  (ii)  when  there  shall be  consummated  any  consolidation,
         merger,  reorganization or similar corporate  transaction (a "Corporate
         Transaction")  of the  Member  (A)  in  which  the  Member  is not  the
         continuing or surviving  entity (other than a  consolidation  or merger
         with a wholly  owned  subsidiary  of the  Member in which all shares of
         common  stock  of such  Member  outstanding  immediately  prior  to the
         effectiveness  thereof  are  changed  into or  exchanged  for the  same
         consideration),  (B)  pursuant to which the common stock of such Member
         would be converted  into cash,  securities or other property or (C) any

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         transaction or series of related  transactions  in which the holders of
         the common stock of the Member  immediately  prior to such  transaction
         (or the first of such transactions, if a series) does not hold at least
         a majority of the total  voting  power of all classes of capital  stock
         entitled to vote  generally  in the election of directors of the Member
         immediately  after such transaction (or the last of such  transactions,
         if a series), in each case, other than a Corporate  Transaction that is
         a  Qualifying  Corporate  Transaction;  (iii) when any  person,  or any
         persons acting  together which would  constitute a "group" for purposes
         of Section  13(d) of the  Securities  Exchange Act of 1934, as amended,
         together  with  any  affiliates  thereof,  shall  beneficially  own (as
         defined in Rule 13d-3 under the Exchange Act) at least 50% of the total
         voting power of all classes of capital stock of the Member  entitled to
         vote  generally in the election of directors of the Member  (unless the
         "acquisition" is deemed to have occurred  indirectly solely as a result
         of the  completion of a Corporate  Transaction  that is not a Change in
         Control  pursuant to clause (ii)  above);  (iv) when at any time during
         any consecutive  two-year  period,  individuals who at the beginning of
         such period  constituted the Board of Directors of the Member (together
         with any new  directors  whose  election by such Board of  Directors or
         whose  nomination  for election by the  stockholders  of the Member was
         approved  by a vote of 75% of the  directors  then  still in office who
         were either directors at the beginning of such period or whose election
         or nomination  for election was  previously so approved)  cease for any
         reason to constitute a majority of the Board of Directors of the Member
         then in office;  or (v) when the Member is  liquidated  or dissolved or
         adopts a plan of liquidation or  dissolution.  A "Qualifying  Corporate
         Transaction" shall be any Corporate Transaction in which the holders of
         common  stock  of  the  Member  immediately  prior  to  such  Corporate
         Transaction  have,  directly or indirectly,  at least (i) a majority of
         the total voting power of all classes of capital stock entitled to vote
         generally in the election of directors of the  continuing  or surviving
         entity  immediately  after such Corporate  Transaction in substantially
         the same  proportion  as their  ownership  of common  stock before such
         transaction  or (ii) 25% of the total  voting  power of all  classes of
         capital stock  entitled to vote  generally in the election of directors
         of the continuing or surviving entity  immediately after such Corporate
         Transaction in substantially  the same proportion as their ownership of
         common  stock before such  transaction  and, in the case of this clause
         (ii), the other Member has given a CIC Approval. A "CIC Approval" shall
         be given by the other  Member if such  Member  cannot  conclude in good
         faith and in its reasonable judgment after discussions with the parties
         to the Corporate  Transaction  that such Corporate  Transaction and any
         resulting  change in management  or leadership is reasonably  likely to
         disrupt  or delay the  commercialization  of  products  by the LLC,  or
         fundamentally  alter  the  approach,  philosophy  or vision of the LLC,
         giving due  regard to the  importance  of  good-faith  cooperation  and
         collaboration  and  single-mindedness  of purpose  between  CYTOGEN and
         Progenics  which has formed the basis for the  collaboration  initially
         contemplated  by  the  parties  hereto.  A  CIC  Approval  may  not  be
         unreasonably  withheld,  and failure of the other  Member to give a CIC
         Approval may be contested in arbitration pursuant to Article 10 hereof.
         In the event of a Qualifying  Corporate  Transaction,  such acquirer or


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         surviving  entity  shall  have the  rights of such  Member,  including,
         without  limitation,  voting and marketing rights.  Notwithstanding the
         foregoing,  a Change of Control shall not include any  transaction  the
         purpose of which is to  reorganize  the Member's  corporate  structure,
         reincorporate the Member in another jurisdiction or undertake any other
         action  which  does  not have  the  purpose  or  effect  of  materially
         affecting  the  ownership  and/or  control of the Member at the time of
         such transaction.

                  (d) The parties  desire for the  foregoing  provisions of this
         Section  6.1 to be  interpreted  as  necessary  to give effect to their
         intent that no Interest be transferred  indirectly,  in one or a series
         of  transactions,  without giving effect to the ownership  consequences
         described in this Agreement.

5.       Section 9.2 of the Agreement is hereby amended to read as follows:

                  9.2 Diligence Obligations.  CYTOGEN shall use diligent efforts
         to market Licensed Products in the North American Territory  ("Diligent
         Efforts"). For purposes of the foregoing, "Diligent Efforts" shall mean
         carrying out such obligation in a commercially reasonable and sustained
         manner  consistent  with the  efforts a  commercial  enterprise  in the
         pharmaceutical  industry  would  devote to a product of similar  market
         potential,  profit  potential or strategic value resulting from its own
         research  efforts  to which such party has  exclusive  rights  based on
         conditions then prevailing. Diligent Efforts requires: (i) developing a
         strategic  plan for product launch and  subsequent  market  penetration
         with  defined  objectives;  (ii)  establishing  systems  and  protocols
         reasonably  designed  to  achieve  such  objectives;  (iii)  allocating
         appropriate  resources  to support  such  systems  and  protocol;  (iv)
         promptly  assigning  responsibility  for  executing  all  phases of the
         Marketing  Plan to  specific  employees  who are held  accountable  for
         discharging their assigned  responsibilities;  and (v) monitoring on an
         ongoing basis the  execution of the Marketing  Plan and its success and
         making  such  changes as are  warranted  by market  and/or  operational
         conditions.  CYTOGEN  shall provide the LLC with access to all relevant
         records and personnel, during normal business hours and with reasonable
         advance notice,  under customary  confidentiality  conditions,  for the
         purpose of determining the  utilization by CYTOGEN of Diligent  Efforts
         to commercialize Licensed Products.

6.       Section 9.7 of the Agreement is hereby amended to read as follows:

                  9.7  Non-Transferability  of  Rights.  The  product  marketing
         rights  granted by the LLC pursuant to this  Agreement  are personal in
         nature to the party to whom they are granted  and,  except as otherwise
         provided in Section  6.1, are not  transferable  or  licensable  in any
         manner  whatsoever by such party to whom they are granted  hereby,  nor
         are such product  marketing rights otherwise  exercisable by any person
         or entity  other than the party to which such rights have been  granted
         hereby.

7.       Section 11.3 of the Agreement is hereby amended to read as follows:


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                           11.3  Entire  Agreement;  Amendment.  This  Agreement
          contains a complete  statement of the  arrangements  among the Members
          with respect to the Company and  supersedes  all prior  agreements and
          understandings  among them with  respect to the Company and may not be
          amended except by unanimous written agreement of the Members.

8.       Section 11.12 of the Agreement is hereby amended to read as follows:

                           11.12 Assignability.  Except as otherwise provided in
         Section 6.1 hereof, neither this Agreement nor any Interest,  including
         the right,  directly or  indirectly,  to receive  distributions  or any
         other  economic,  voting or  management  rights or interests , shall be
         assignable by either Member  without the written  consent of the other,
         and any  attempted  assignment  without such consent  shall be null and
         void. This Agreement shall be binding upon the successors and permitted
         assigns of the Members. Any such successor or permitted assign shall be
         subject  to the same  rights and  obligations  as the  original  Member
         hereunder.

9. This Amendment  Number 1 to the Agreement shall be deemed an amendment of the
Agreement  for  purposes  of Section  11.3 the  Agreement.  Except as  expressly
provided in this Amendment  Number 1, all provisions of the Agreement  remain in
full force and effect.

10. This Amendment  Number 1 supercedes all prior  agreements,  discussions  and
correspondence between Progenics, CYTOGEN and the Company concerning the subject
matter of said provisions.

         IN WITNESS  WHEREOF,  the parties have executed this Amendment Number 1
as of the date and year first written above.

Progenics Pharmaceuticals, Inc.                  PSMA Development Company LLC


By:  /s/ Ronald J. Prentki                       By: /s/ Paul J. Maddon
    ---------------------------                     ----------------------------
       Ronald J. Prentki                               Paul J. Maddon
       President                                       Managing Representative

CYTOGEN Corporation


By:  /s/ H. Joseph Reiser                        By:  /s/ H. Joseph Reiser
    ---------------------------                      ---------------------------
       H. Joseph Reiser                                H. Joseph Reiser
       President and CEO                               Managing Representative



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