<PAGE>   1

                                                                EXHIBIT 10.18(a)

                              EMPLOYMENT AGREEMENT

         EMPLOYMENT AGREEMENT made as of the first day of August, 1996, between
Introgen Therapeutics, Inc., a Delaware corporation (the "Company"), and David
G. Nance (the "Employee").

         1. Employee Duties. The Company hereby employs the Employee as its
President for the Term (as hereinafter defined), with such responsibilities and
duties as the Board of Directors may from time to time determine consistent with
such job title. The Employee shall devote such time as is reasonably necessary
to the performance of his responsibilities and duties hereunder provided,
however, that nothing herein contained shall restrict the Employee from serving
as a consultant to and/or acting as a director or officer of other entities not
in competition with the Company in the field of in vivo gene therapy for the
development of p53 gene replacement, K-ras inhibition or C-CAM tumor suppressor
gene therapy. The Company acknowledges that as of the date of this Agreement
Employee has numerous outside business interests and activities, including
serving as president of Domecq Technologies, Inc., president of Technology
Capital Corporation, trustee of several private and charitable trusts and
foundations, president of DGX Corporation, Domecq Sebastian Holdings, partner
(as Trustee) of Texas Biomedical Development Partners and its successors,
service in several outside directorships and management of assets including but
not limited to securities and real estate. Through those entities and otherwise
Employee is and will continue to be active outside the Company in the areas of
biotechnology, pharmaceuticals, research and development, medical technology,
and healthcare. The Company agrees that Employee may continue these and other
similar activities throughout the term, provided that such activities do not
violate the provisions of this Agreement.

         2. Term of Agreement. The term of employment (the "Term") shall
commence on the date first written above, and end on July 31, 1998, unless
renewed in accordance with the terms hereof.

         3. Compensation. Base Salary. Employee shall receive an aggregate base
salary at the rate of $225,000 per annum during the Employment Term; provided,
however, that should the Company become a publicly traded corporation then the
Company and the Employee agree to negotiate in good faith and adjust the amount
of salary and other compensation so that, effective upon the first anniversary
hereof, they will be consistent with those paid by other public companies of
similar valuations and promise to their chief executive officers, and to
reflect the additional responsibilities and duties of a chief executive officer
of a public company. Installments of base salary shall be paid not less
frequently than bi-weekly. Employee may be paid bonuses (in stock, cash or
other consideration), from time to time as determined by the Board of Directors.

<PAGE>   2
         4. Other Fringe Benefits. Employee shall receive the following
benefits during the Term of Employment: Pension, profit sharing plan, 401K plan
and stock option plan participation, comprehensive health, accident, major
medical, disability and life insurance protection in accordance with the
general policies of the Company as in effect from time to time.

         5. Reimbursement of Expenses. The Company shall reimburse Employee for
all reasonable, ordinary and necessary expenses incurred by him in the
performance of his duties hereunder, provided that Employee accounts to the
Company therefore in the manner prescribed by the Company for reimbursement of
Employee's expenses.

         6. Vacation. Employee shall be entitled to a reasonable number of
vacation days each year, and in any event shall receive at least as many
vacation days as allowed any other Company Employee. Unused vacation days may
be accumulated from year to year.

         7. Term of Employment. Term of Employment shall mean the period
commencing on the date provided in Section "2" and ending on the earliest to
occur of:

            (i) The death of Employee;

            (ii) The Termination Date or such later date as may be set by
mutual consent expressed in a writing signed by both parties hereto;

            (iii) Termination for cause pursuant to Section 8 of this
Employment Agreement; or

            (iv) Termination by Employee pursuant to Section 8.

         8.2 Termination of Employment.

         8.3 Termination for Cause. The Company may terminate Employee's
employment only for "Cause." A termination for Cause is a termination evidenced
by a finding adopted in good faith by the Board that Employee (i) willfully and
continually failed to substantially perform his duties with the Company (other
than a failure resulting from Employee's incapacity due to illness, physical or
mental disability or other incapacity) and such failure continues after the
Board has given written notice that Employee has failed to perform his duties,
(ii) has been convicted of a felony, (iii) has breached this Agreement in any
material respect if such breach is not cured or remedied reasonably promptly
after the Board has given written notice to Employee providing a reasonable
description of the breach, or (iv) engaged in conduct constituting willful
malfeasance in connection with his employment which is materially and
demonstrably injurious to the Company and


                                       2
<PAGE>   3
its subsidiaries taken as a whole. No act, or failure to act, on Employee's
part, shall be considered "willful" for purposes of (i) or (iv) above unless he
has acted or failed to act with an absence of good faith and without a
reasonable belief that his action or failure to act was in the best interests
of the Company. Notwithstanding anything contained in this Agreement to the
contrary, no failure to perform by Employee after Notice of Termination (as
hereinafter defined) is given by Employer shall constitute Cause for purposes
of this Agreement. Termination for Cause shall be by action of the Board after
giving Employee and his legal advisor an opportunity to meet with the Board,
contest the basis for termination, and to demonstrate that Employee's continued
employment is in the best interests of the Company.

         8.2 Termination by Employee. In the event that Employee's
responsibilities, job title, or authority are materially altered by Employer,
then Employee may terminate this Agreement at any time.

         8.3 Effect of Termination.

              (i) If Employee's employment hereunder shall be terminated by the
Company for cause pursuant to Section 8 hereof, this Agreement shall forthwith
terminate except that Employee's obligations under Section 10 shall continue
unaffected.

             (ii) If Employee's employment hereunder shall be terminated by the
Company other than for cause, Employee's obligations under Section 10 shall
terminate and the Company shall pay to the Employee all compensation otherwise
payable to the employee hereunder to the same extent as if this Agreement had
not been terminated.

            (iii) If Employee's employment hereunder is terminated by Employee
pursuant to Section 8.2, then all of Employee's obligations and compensation
hereunder shall cease except that Employee shall be entitled to receive all
compensation accrued under Section 3, 4 and 5 but unpaid as of the date of
termination.

         9. Death of Employee. If Employee's employment hereunder shall
terminate because of his death, this Agreement shall forthwith terminate,
except that Employee's personal representative shall be entitled to receive all
compensation accrued in favor of Employee under Sections 3, 4, and 5 but unpaid
as of the date of death. All rights of Employee's personal representative to
receive any further compensation hereunder or under any other plan, arrangement
or procedure of the Company shall terminate, to the extent not theretofore
vested, except for any rights which arise by virtue of Employee's death under
any such plan, arrangement or procedure.


                                       3
<PAGE>   4
     10.  Non-Compete.

     10.1 Non-Competition. Employee agrees that he will not during the Term of
this Agreement engage in, or otherwise directly or indirectly be employed by,
or act as a consultant or lender to, or be a director, officer, employee, owner
or partner of, any other business or organization that is now or shall
hereafter be competing with the Company in the field of in vivo gene therapy
for the development of P53 gene replacement, K-ras inhibition, or C-CAM tumor
suppressor gene therapy within any geographical area where the Company is doing
business. Subject to the foregoing, the Company acknowledges that Employee may
continue to serve in the capacities and participate in the activities outside
the Company described in Section 1 hereof and similar capacities and
activities, and that such participation will not constitute prohibited
competition hereunder.

     10.2 Nondisclosure of Confidential Information; Non-Competition. (a)
Employee shall not, without the prior written consent of the Company, divulge,
or disclose to any other person, firm, partnership, corporation or other entity
any Confidential Information pertaining to the business of the Company, except
(i) while employed by the Company, in the business of and for the benefit of the
Company, or (ii) when required to do so by a court of competent jurisdiction, by
any governmental agency having supervisory authority over the business of the
Company, or by any administrative body or legislative body (including a
committee thereof) with purported or apparent jurisdiction to order Employee to
divulge, or disclose such information. For purposes of this Section 10(a),
"Confidential Information" shall mean non-public information concerning the
Company's financial data, strategic business plans, product development (or
other proprietary product data), customer lists, marketing plans or other
non-public, proprietary and confidential information of the Company, in each
case which is not otherwise available to the public.

     10.3 Branch of Section 10. Since a breach of the provisions of this
Section 10 could not be adequately compensated by money damages, the Company
shall be entitled, in addition to any other right and remedy available to it,
to an injunction restraining such breach or threatened breach. Employee agrees
that the provisions of this Section 10 are necessary and reasonable to protect
the Company in the conduct of its business. If any restriction contained in
this Section 10 shall be deemed to be invalid, illegal, or unenforceable by
reason of the extent, duration, or geographical scope thereof, or otherwise,
than the court making such determination shall have the right to reduce such
extent, duration, geographical scope, or other provisions hereof, and in its
reduced form such restrictions shall then be enforceable in the manner
contemplated hereby.

                                       4
<PAGE>   5

         11. Warranties and Representations of the Employee. The Employee
warrants and represents that the Employee is not subject to any agreement,
contract, judgment, decree or limitation the effect of which would prohibit,
limit or otherwise restrict the employment of the Employee by the Company
pursuant to the terms of this Agreement.

         12. Notices. All notices, requests, consents and other communications
required or permitted to be given hereunder, shall be in writing and shall be
deemed to have been duly given if delivered personally or sent by prepaid
telegram, or mailed first-class, postage prepaid, by registered mail (notices
sent by telegram or mailed shall be deemed to have been given on the date
sent), as follows (or to such other address as either party shall designate
by notice in writing to the other in accordance herewith):

               If to the Employee:

               David G. Nance
               25 St. Stephens School Road
               Austin, Texas 78746

               If to the Company:

               Introgen Therapeutics, Inc.
               301 Congress Avenue, Suite 1850
               Austin, Texas 78701

         13. Governing Law. This Agreement shall be governed by and construed
and enforced in accordance with the local laws of the State of Texas applicable
to agreements made and to be performed entirely in such state. The parties
agree that this Agreement is performable in Travis County, Texas, and that
venue of any action concerning this Agreement shall lie in Travis County, Texas.

         14. Heading and Captions. The section headings contained herein are
for reference purposes only and shall not in any way affect the meaning or
interpretation of this Agreement.

         15. Entire Agreement. This Agreement sets forth the entire agreement
and understanding of the parties relating to the subject matter hereof, and
supersedes all prior agreements, arrangements, and understandings, written or
oral, relating to the subject matter hereof. No representation, promise or
inducement has been made by either party that is not embodied in this
Agreement, and neither party shall be bound by or liable for the alleged
representation, promise or inducement not so set forth.

         16. Amendments To Waivers. This Agreement may be amended, modified,
superseded, cancelled, renewed or extended and the terms or covenants hereof
may be waived only by a written instrument

                                       5


<PAGE>   6
executed by both of the parties hereto, or in the case of a waiver, by the
party waiving compliance. the failure of either party at any time or times to
require performance of any provision hereof shall in no manner affect the right
at a later time to enforce the same. No waiver by either party of the breach of
any term or covenant contained in this Agreement, whether by conduct or
otherwise, in any one or more instances, shall be deemed to be, or construed
as, a further or continuing waiver of any such breach, or a waiver of the
breach of any other term or covenant contained in this Agreement.


                                       6
<PAGE>   7

         IN WITNESS WHEREOF, the parties have enacted this Agreement as of the
date first above written.

             COMPANY:             INTROGEN THERAPEUTICS, INC.

                                  By: /s/ John N. Kapoor
                                    ------------------------------
                                      John N. Kapoor, Chairman


             EMPLOYEE:                /s/ David G. Nance
                                    ------------------------------
                                      David G. Nance


                                       7