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                                                                EXHIBIT 10.20(b)


                                 ADDENDUM NO. 1
                    TO COLLABORATION AGREEMENT (p53 PRODUCTS)


         This Addendum No. 1 ("Addendum") is made by and between Introgen
Therapeutics, Inc. ("Introgen") and Rhone-Poulenc Rorer Pharmaceuticals Inc.
("RPRP") as of January 23, 1996 ("Effective Date"), to that certain
Collaboration Agreement (p53 Products) dated as of October 7, 1994 between
Introgen and RPRP (the "Collaboration Agreement").

                               B A C K G R O U N D

         A. Section 5.2 of the Collaboration Agreement provides, among other
things, that RPRP and Introgen shall each have the right to develop the
Collaboration Products within the Field in the Co-Exclusive Territory, which
includes Japan (among other countries).

         B. Section 7.2 of the Collaboration Agreement provides that RPRP and
Introgen shall each have the co-exclusive right to market, sell and distribute
Collaboration Products in the Co-Exclusive Territory.

         C. To expedite commercialization of the initial Collaboration Products,
RPRP and Introgen desire to initiate "first-in-human" clinical trials in Japan
for two (2) initial Collaboration Products on a cooperative basis, on the terms
and conditions set forth below.

                                A G R E E M E N T

         NOW, THEREFORE, in consideration of the mutual covenants and premises
herein contained, the parties hereto, intending to be legally bound, agree as
follows:

         1. Except as otherwise expressly defined herein, capitalized terms
shall have the meaning defined in the Collaboration Agreement.

         2. RPRP and Introgen agree to conduct in accordance with this Addendum
a [*] in Japan for the adenoviral P53 Collaboration Product that is currently
the subject of clinical trials in the United States (the "P53/AV Product"). In
addition, the parties agree to conduct [*] in Japan for [*] Collaboration
Product [*], by the Development Committee under the Collaboration Agreement [*].
It is understood that RPRP may propose to the Development Committee [*], and
Introgen's representatives on the Development Committee will not unreasonably
withhold their consent to such proposal. If the [*] substantially identical to
this Addendum. Together, the [*] to be conducted hereunder are referred to below
as the "Initial Japanese Clinical Development." [*]

      [*] Certain information on this page has been omitted and filed
          separately with the Commission. Confidential treatment has been
          requested with respect to the omitted portions.

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         [*]

         3. Promptly following the Effective Date, RPRP and Introgen shall form
a development committee (the "Japanese Development Committee"), which will
consist of three (3) representatives from each of RPRP and Introgen. Introgen
and RPRP may each replace its Japanese Development Committee representatives at
any time, with written notice to the other party. The Japanese Development
Committee shall oversee, review and coordinate the Initial Japanese Clinical
Development. All decisions of the Japanese Development Committee shall be by
absolute majority vote. RPRP's lead representative shall chair all meetings of
the Japanese Development Committee.

                  (a) The Japanese Development Committee shall meet every four
         (4) months, or more often as agreed by the parties, in Tokyo, Japan, or
         such other location as the parties agree. The parties agree that at
         least one meeting of the Japanese Development Committee per full
         calendar year will be held at Introgen's facilities in Houston. At its
         meetings, the Japanese Development Committee will [*].

                  (b) RPRP shall be responsible for preparing reasonably
         detailed plans and budgets under which the Initial Japanese Clinical
         Development will be carried out. Upon approval of the Japanese
         Development Committee, and subject to such approval, such plans and
         budgets shall be the "Japanese Development Plan and Budget" for all
         purposes of this Addendum.

                  (c) Promptly following the Effective Date, RPRP shall prepare
         and provide to the Japanese Development Committee a proposed Japanese
         Development Plan and Budget for the P53/AV Product for which the
         Initial Japanese Clinical Development will be conducted. [*] The
         Japanese Development Committee shall review such proposal as soon as
         possible and shall establish and approve no later than October 31 of
         such year the final Program Plan and Budget for the next succeeding
         year. The Japanese Development Committee shall review the Japanese
         Development Plan and Budget


         [*] Certain information on this page has been omitted and filed
             separately with the Commission. Confidential treatment has been
             requested with respect to the omitted portions.

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         on an ongoing basis and may make changes to the Japanese Development
         Plan and Budget then in effect; provided, however, the Japanese
         Development Plan and Budget in effect at any time shall not be modified
         except as approved by the Japanese Development Committee.

     4. RPRP shall conduct the Initial Japanese Clinical Development in
accordance with the Japanese Development Plan and Budget and shall use its
reasonable diligent efforts to meet the time schedules contemplated therein. It
is understood that, subject to the Japanese Development Plan and Budget and the
control of the Japanese Development Committee, RPRP will be responsible for the
clinical management and supervision of the clinical trials conducted under this
Addendum.

                  (a) RPRP agrees to keep Introgen fully informed of the

         progress of the Initial Japanese Clinical Development conducted
         hereunder. [*] RPRP agrees to provide Introgen English translations of
         information generated in connection with the Initial Japanese Clinical
         Development. To the extent permitted by law or regulation, all
         regulatory approvals and filings with respect to the Initial Japanese
         Clinical Development will be made jointly in the names of RPRP and
         Introgen, or in such other manner as the parties agree, and as
         requested by Introgen, RPRP shall use reasonable diligent efforts to
         take such actions as may be necessary or appropriate to enable Introgen
         to transfer or extend to third parties co-equal benefits in such
         approvals and filings.

                  (b) [*] All trial protocols and regulatory submissions with
         respect to the Initial Japanese Clinical Development, and all
         agreements entered into by RPRP with respect to the conduct of the
         Initial Japanese Clinical Development, shall be subject to the review
         and approval of the Japanese Development Committee.

                  (c) Without limiting the foregoing, RPRP shall periodically,
         and not less often than semi-annually during the term of this Addendum,
         provide the Japanese Development Committee with a written report
         summarizing the progress of the Initial Japanese Clinical Development
         with respect to each Initial Japanese Product during the preceding two
         calendar quarters.

         5. Subject to the remainder of this Section 5, RPRP will advance the
costs associated with the Initial Japanese Clinical Development. Introgen will
reimburse [*] of RPRP's out-of-pocket costs approved by the Japanese Development
Committee that are paid to third parties to conduct clinical trials under the
Initial Japanese Clinical Development, and [*], supplied under paragraph 6 below
for use in such Initial Japanese Clinical Development ("Reimbursable Costs"),
regardless of the trial outcome. [*], no later than the date that is [*] after
completion of all clinical trials conducted hereunder (the "Due Date"). At
Introgen's option, however, Introgen may [*].

         [*] Certain information on this page has been omitted and filed
             separately with the Commission. Confidential treatment has been
             requested with respect to the omitted portions.

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In the event of such election, Introgen shall issue [*]. For the avoidance of
doubt, following such issuance, [*]. In the event [*] or if [*], then so long as
[*]. The foregoing amounts to be reimbursed to RPRP by Introgen shall bear
interest at the [*], as published from time to time by Bank of America NT&SA in
San Francisco, California, from the date the amounts are paid by RPRP until the
date such amounts are reimbursed by Introgen. It is understood that [*].
Notwithstanding anything contained in this Addendum to the contrary, in the
event that [*], and (ii) Introgen and/or the third party shall thereafter be
required to pay on an ongoing basis [*] of the Reimbursable Costs associated
with the Initial Japanese Clinical

         [*] Certain information on this page has been omitted and filed
             separately with the Commission. Confidential treatment has been
             requested with respect to the omitted portions.


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Development for such Initial Japanese Product and (iii) Introgen and/or the
third party shall reimburse RPRP, on the Due Date in accordance with the
foregoing provisions of this Section 5, any Reimbursable Costs (together with
accrued interest thereon) accrued prior to the date of such Partnering
Arrangement.

         6. Introgen shall supply [*] quantities of the P53/AV Product to be
used in the Initial Japanese Clinical Development, on the same terms [*].

         7. Section 2.5 of the Collaboration Agreement shall be deemed to apply
to all publications of results of the Initial Japanese Clinical Development
(except that all references to the Development Committee shall be deemed to
refer to the Japanese Development Committee), but neither of the last two
sentences of Section 5.4.2, nor any of Section 5.4.3 of the Collaboration
Agreement, shall apply to preclinical data, clinical data or regulatory filings
generated or made in connection with the Initial Japanese Clinical Development
conducted hereunder.

         8. RPRP agrees to use its reasonable diligent efforts to conduct and
complete as expeditiously as possible the Initial Japanese Clinical Development.

         9. [*]

         10. The term of this Addendum shall continue until completion of each
[*] within the Initial Japanese Clinical Development conducted hereunder;
provided that this Addendum may be extended by the parties for such longer
period as the parties may mutually agree in writing.

         11. This Addendum shall be deemed a part of the Collaboration
Agreement. Except as otherwise expressly provided in this Addendum, all rights
and obligations with respect to Collaboration Products in Japan will be as set
forth in the Collaboration Agreement. Following the expiration or termination of
this Addendum, all obligations of the parties under this Addendum, except those
under Paragraphs 4(a), 5 and 7 above, shall terminate and be of no further force
or effect. It is understood that nothing in this Addendum shall be deemed to
[*].

                                          RHONE-POULENC RORER
INTROGEN THERAPEUTICS, INC.               PHARMACEUTICALS INC.

By: /s/ David G. Nance                    By:    K. R. Pina
    ---------------------------------          --------------------------------
         DAVID NANCE                               Vice President &
         President                                 General Counsel

Date:  01-23-1996                          Date: 01-19-1996
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         [*] Certain information on this page has been omitted and filed
             separately with the Commission. Confidential treatment has been
             requested with respect to the omitted portions.


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