EXHIBIT 10.21


                  THIRD AMENDMENT TO STOCK PURCHASE AGREEMENT


THIRD AMENDMENT TO STOCK PURCHASE AGREEMENT (the "Amendment"), dated July 28,
1997, between PHARMACEUTICAL RESOURCES, INC., a New Jersey corporation (the
"Company"), and CLAL PHARMACEUTICAL INDUSTRIAL LTD., a corporation formed under
the laws of the State of Israel, (the "Purchaser").

WHEREAS, the Company and the Purchaser entered into a Stock Purchase Agreement,
dated March 25, 1995, as amended pursuant to Amendment No. 1 to Stock Purchase
Agreement, dated May 1, 1995, and Amendment No. 2 to Stock Purchase Agreement
(as amended, the "SPA"); and

WHEREAS, a subsidiary of the Company is acquiring all of the interests in the
Joint Venture (as defined in the SPA) held by a subsidiary of the Purchaser;

WHEREAS, incident to such acquisition, the Company and the Purchaser desire to
amend certain terms of the SPA and the Registration Rights Agreement between the
Company and the Purchaser, dated May 1, 1995, and desire to set forth their
mutual agreements with respect thereto.

NOW, THEREFORE, in consideration of the premises and of the mutual covenants set
forth herein, the parties hereto agree as follows:

1.   Definitions.  Unless otherwise defined herein, capitalized terms used
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herein shall have the same meanings as in the SPA.

2.   New Shares.
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     2.1  The Company shall execute and deliver to the Purchaser a certificate
     representing 186,000 shares of Common Stock (the "New Shares") promptly
     following approval for listing of the New Shares by The New York Stock
     Exchange, provided that, in the event that the Company shall not deliver
     the New Shares by the 42nd day following the execution and delivery of this
     Amendment, (i) PRI Research, Inc. hereby agrees that principal amount of
     the Non-Recourse Secured Promissory Note, dated the date hereof, of PRI
     Research, Inc. shall be increased by an amount equal to the product of the
     closing price of a share of Common Stock on the trading day prior to the
     execution and delivery of this Amendment multiplied by 186,000 and (ii) the
     Company's obligation to deliver the New Shares hereunder shall terminate.
     The Company shall file an application for the listing of the New Shares
     with The New York Stock Exchange promptly following the execution and
     deliver of this Amendment. The Purchaser shall pay to the Company the sum
     of $1,860 (representing the par value of the New Shares) upon the delivery
     of the New Shares to the Purchaser. If the New Shares shall not be
     delivered, the other 

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     agreements executed and delivered by the Company, the Purchaser and their
     respective affiliates on the date hereof or contemplated thereby shall
     remain in full force and effect, except for the Non-Recourse Secured
     Promissory Note which shall be modified as stated herein.

     2.2  Simultaneous with the execution and delivery of this Amendment, the
     Purchaser shall deliver to the Company the original Warrant and Additional
     Warrant (or an affidavit of lost security and indemnification agreement in
     the event the original security is misplaced or destroyed). The New Shares
     shall be issued, or the principal amount of the Non-Recourse Secured
     Promissory Note, dated the date hereof, of PRI Research, Inc. shall be
     increased, in exchange for the surrender and cancellation of the Warrant
     and the Additional Warrant.

     2.3  The Company and the Purchaser hereby agree that references to
     "Securities" in the SPA shall also include and refer to the New Shares.

3.   Third party transactions.
     -------------------------

     3.1  In Section 10 of the SPA the terms "60 days" and "60-day period"
     wherever they appear shall be amended to read "30 days" and "30 day
     period", respectively.

     3.2  It is hereby clarified that a bona fide offer made for more than 10%
     of PRI's securities, but which could result by its express terms in the
     acquisition of more than 50% of PRI's outstanding voting securities, shall
     be deemed a Third Party Transaction for the purposes of Section 10 of the
     SPA.

4.   Acquisitions and Dispositions of Securities.
     --------------------------------------------

     4.1  The first sentence of Section 11.1(a) of the SPA shall be amended in
     its entirety as follows:

          (a) During the period ("Consent Period") commencing on May 1, 1995 and
     terminating six months after the date on which the Purchasers' rights shall
     terminate under Section 10.1 hereof, the Purchaser shall not sell, assign,
     pledge, transfer or otherwise dispose of (collectively, a "Transfer") any
     Securities (as hereinafter defined) without the written consent of the
     Company (which may be granted or withheld in its sole discretion) unless
     such Securities (i) shall be registered under the Securities Act and
     applicable state securities laws, (ii) shall be sold in brokers'
     transactions pursuant to Rule 144 promulgated under the Securities Act,
     (iii) shall be sold or transferred in connection with a Third Party
     Transaction or any other transaction that has been approved by a majority
     of the members of the Board (exclusive of those members appointed by the
     Purchaser pursuant to Section 7.2 hereof),  (iv) shall be sold or
     transferred in any transaction which shall comply with the Securities Act
     and applicable state securities laws, in accordance with Section 11.1(b)
     hereof or, (v) a Transfer of all Securities owned at the time of Transfer
     by the Purchaser if the Board has written notice of such Transfer and the
     Company's Board of Directors does not reject the transferee, it being
     agreed that the company's Board of Directors may 

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     only reject such transferee (but subject to Section 11.1(c) hereof) if such
     entity is asserted in good faith and then demonstrated by the Board (acting
     in good faith) to be a competitor or a party with a demonstrated adverse
     interest to the Company (the Board to act within 7 U.S. business days of
     the Company's receipt of a notice regarding such contemplated Transfer).

     4.2  The following shall be inserted as Section 11.1(e) of the SPA:

          (e) Notwithstanding the provisions of Section 11.1 hereof, the
     Purchaser may Transfer 90% or more of the Common Stock then beneficially
     owned by the Purchaser to a bona fide purchaser if the Company's Board of
     Directors does not reject the transferee as provided below (a "Permitted
     Transfer"); provided, however, that the Purchaser shall not be entitled to
     request or consummate a Permitted Transfer if, at the time of the
     Purchaser's request to the Company for its consent to the Permitted
     Transfer, the Purchaser shall have Transferred more than 290,000 shares of
     Common Stock in any 365-day period or shall have Transferred an aggregate
     of more than 586,000 shares of Common Stock since May 1, 1995. The
     Purchaser will advise the Company of the beneficial owner of the proposed
     transferee.  The Company's Board of Directors may reject the tranferee if
     such entity is asserted in good faith and then demonstrated by the Board
     (acting in good faith) to be a competitor or a party with a demonstrated
     adverse interest to the Company (the Board to act within 7 U.S. business
     days of the Company's receipt of a notice regarding such contemplated
     sale).

5.   Assignment.
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     (a) Section 16.2 of the SPA shall be amended in its entirety as follows:

          16.2  Assignment.  All terms and provisions of this Agreement shall be
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     binding upon and inure to the benefit of the parties hereto and their
     respective successors and permitted assigns, but neither this Agreement nor
     any of the rights, interests or obligations hereunder may be assigned by
     any party hereto without the prior written consent of the other party;
     provided, that, (a) the Purchaser may assign its rights under this
     Agreement, in whole or in part, to any subsidiary or related entity "Hevra
     Kshura" of the Purchaser, within the meaning of the Israel Securities Act
     5728-1968, as amended, so long as such (i) subsidiary or related party
     shall assume and agree to be bound by all of the Purchaser's obligations
     hereunder and (ii) the Purchaser shall not be relieved of its primary
     liability to the Company for all of the Purchaser's obligations set forth
     herein and (b) the Purchaser may assign all, but not less than all, of its
     rights under this Agreement to any person or entity pursuant to a Permitted
     Transfer so long as the transferee thereof shall assume and agree to be
     bound by all of the Purchaser's obligations hereunder (a "Permitted
     Assignment").

     (b) The Company and the Purcahser hereby acknowledge and agree that, for
     the purposes of the Rights Agreement, between the Company and Midlantic
     Bank, N.A., dated August 6, 1991, as amended, only transferees and
     assignees of Purchaser pursuant to Sections 16.2(a) or (b) of the
     Agreement, as amended, 

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     shall constitute "permitted assigns" of Clal Pharmaceutical Industries Ltd.
     under Section 1(a)(v) of such Rights Agreement. Purchaser shall inform all
     transferees of Securities of this provision.

6.   Registration Rights Agreement.  Section 8.1 of the Registration Rights
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Agreement shall be amended in its entirety as follows:

     8.1  Assignment.  All terms and provisions of this Agreement shall be
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     binding upon and inure to the benefit of the parties hereto and their
     respective successors and permitted assigns, but neither this Agreement nor
     any of the rights, interests or obligations hereunder may be assigned by
     any party hereto without the prior written consent of the other party;
     provided, that, (a) the Holder may assign this Agreement to any permitted
     assignee under Section 16.2(a) of the Stock Purchase Agreement without the
     Company's written consent so long as (i) such assignee shall agree to
     assume and agree to be bound by all of the Holder's obligations hereunder
     and (ii) the Holder shall not be relieved of its primary liability to the
     Company for all of the Holder's obligations set forth herein and (b) the
     Holder may assign all, but not less than all, of the rights under this
     Agreement to a person or entity pursuant to a Permitted Transfer as defined
     in the Stock Purchase Agreement so long as such assignee shall agree to
     assume and agree to be bound by all of the Holder's obligations hereunder.

     The Registration Rights Agreement shall also apply to the New Shares.

This provision shall constitute an amendment of the Registration Rights
Agreement pursuant to Section 8.4 thereof.

7.   Representations and Indemnification.
     ------------------------------------

     7.1  Representations.  The Company and the Purchaser each hereby represent
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and warrant to the other as follows:

     (a) It is a corporation duly organized, validly existing, and in good
     standing under the laws of the jurisdiction of its formation. It has all
     requisite corporate power and authority to conduct its business and to
     enter into and perform its obligations under this Amendment in accordance
     with the terms hereof.

     (b) It has taken all required corporate actions to approve and adopt this
     Amendment. This Amendment constitutes a duly authorized, valid and binding
     agreement on it and enforceable against it in accordance with its terms.
     Each person executing this Amendment on its behalf is duly authorized and
     empowered to do so.

     (c) The execution and delivery of this Amendment and the consummation of
     the transactions as contemplated hereunder (i) do not, and will not,
     violate or conflict with any statute, regulation, judgment, order, writ,
     decree or injunction currently applicable to it or any of its property or
     assets; and (ii) do not, and will not, violate or conflict with its charter
     or By-laws and/or Memorandum and 

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     Articles of Association, or any existing mortgage, indenture, contract,
     licensing agreement, financing statement or other agreement binding on it.

     (d) All required consents and approvals, as well as any approvals or
     consents of any governmental authorities or any other third parties in
     connection with the execution and delivery of this Amendment or the
     performance of the transactions contemplated hereunder, have been obtained
     by it, except for such approvals required under New York Stock Exchange
     rules. No contract or agreement binding upon it restricts its ability to
     fulfill its obligations and responsibilities under this Amendment or to
     carry out the activities contemplated herein.

     (e) It is not a party to or, to the best of its knowledge, threatened with
     any litigation or judicial or administrative proceeding that, if decided
     adversely to it, would delay or preclude the consummation of the
     transactions contemplated in this Amendment or have a material adverse
     effect upon the transactions contemplated hereby.

     7.2  Indemnification.  The Company and the Purchaser each agree to
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indemnify and hold harmless the other and their respective employees, agents and
affiliates against all losses, liabilities, claims, damages, and expenses
(including, but not limited to, reasonable counsel fees) resulting from or
arising out of any actual or alleged misrepresentation or breach by it of any
representation or warranty set forth in Section 8.1 hereof or otherwise set
forth in this Amendment.

8.   Miscellaneous
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     8.1  No Further Amendment.  Except as amended herein, the terms and
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provisions of the SPA and the Registration Rights Agreement are hereby ratified,
confirmed and approved in all respects.

     8.2  Assignment. All terms and provisions of this Amendment shall be
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binding upon and inure to the benefit of the parties hereto and their respective
successors and permitted assigns, but neither this Amendment nor any of the
rights, interest or obligations hereunder may be assigned by any party hereto
without the prior written consent of the other party, other than pursuant to a
Permitted Assignment.


     8.3  Entire Agreement.  This Amendment and the other agreements referred to
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herein or delivered pursuant hereto contain the entire agreement among the
parties with respect to the subject matter hereof and supersedes all prior and
contemporaneous arrangements or understandings with respect thereto.

     8.4  Amendments; Waiver.  This Amendment may not be amended or terminated,
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and no provision hereof may be waived, except pursuant to a written instrument
executed by each of the parties hereto.

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     8.5  Counterparts.  This Amendment may be executed in any number of
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counterparts, and each such counterpart shall be deemed to be an original
instrument, but all such counterparts together shall constitute but one
agreement.

     8.6  Headings.  The headings of the Sections of this Amendment have been
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inserted for convenience of reference only and shall not be deemed to be a part
of this Amendment.

     8.7  Governing Law.  This Amendment shall be governed by and construed in
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accordance with the laws of the State of New York applicable to contracts made
and to be performed wholly therein.

     8.8  Severability.  If any term or provision hereof shall be invalid or
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unenforceable, (a) the remaining terms and provisions hereof shall be
unimpaired, (b) any such invalidity or unenforceability in any jurisdiction
shall not invalidate or render unenforceable such provision in any other
jurisdiction and (c) the invalid or unenforceable term or provision shall be
deemed replaced by a term or provision as determined by a court to be valid and
enforceable and to express the intention of the parties with respect to the
invalid or unenforceable term or provision.

     8.9  Consent to Jurisdiction.  In connection with any dispute which may
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arise under this Amendment or under any other agreement referred to herein, each
of the parties hereby irrevocably submits to, consents to, and waives any
objection to, the jurisdiction of the courts of the State of New York located in
the County of New York or of the United States District Court for the Southern
District of New York, and waives any objection to the laying of venue in such
courts. Each such party admits that any such dispute may be resolved at least as
conveniently in such a court as in any other court, and shall not seek dismissal
or a change of venue on the ground that resolution of such a dispute in any such
court shall not be convenient or in the interests of justice. The Purchaser
hereby appoints Proskauer Rose LLP as its agent upon whom service of process may
be made with the same force and effect as if such service shall have been made
personally upon the Purchaser. The Company hereby appoints Hertzog, Calamari &
Gleason as its agent upon; whom service of process may be made with the same
force and effect as if such service shall have been made personally upon the
Company.


IN WITNESS WHEREOF, each of the undersigned has caused this Third Amendment to
Stock Purchase Agreement to be executed as of the date first written above.

PHARMACEUTICAL RESOURCES, INC.

By:/s/Kenneth I. Sawyer, President
   --------------------------------

CLAL PHARMACEUTICAL INDUSTRIES LTD.

By:             /s/
   --------------------------------

By:            /s/
   --------------------------------

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AGREED AND ACCEPTED AS TO SECTION 2.1 ONLY


PRI - RESEARCH, INC.

By:/s/Kenneth I. Sawyer, President
   --------------------------------

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