Document:

<PAGE>

                                                                   EXHIBIT 10.18

                              CONSULTING AGREEMENT

         THIS AGREEMENT is made and entered into as of this 1st day of January,
2003, ("Effective Date") by and between ILEX(TM) Products, Inc. with place of
business at 4545 Horizon Hill Blvd., San Antonio, Texas 78229 (ILEX Products,
Inc. and its parent and affiliated corporations and partnerships collectively
referred to as "ILEX") and Daniel D. Von Hoff., M.D., and his solely owned legal
entity known as NDA, LLC, located at 4951 N. Avenida de Vizcaya, Tucson, AZ
85718 (hereinafter referred to as "Consultant").

         THE PARTIES AGREE as follows:

         1.       Services

                  ILEX hereby engages Consultant to serve as a consultant to
ILEX, and Consultant hereby accepts such engagement, upon and subject to the
terms and conditions set forth herein.

                  From and following the Effective Date of this Agreement, Dr.
Von Hoff shall update ILEX on a monthly basis as to any other consulting or
advisory board agreements that are made. Dr. Von Hoff warrants and represents to
ILEX that Dr. Von Hoff has no affiliations in any capacity with any persons or
entities other than ILEX, except those listed on EXHIBIT A attached hereto.

                  The parties understand and acknowledge that potential
conflicts or duality of interest, or the appearance of such conflict or duality
of interest, may arise during Dr. Von Hoff's performance of those duties and
services as a result of Dr. Von Hoff's other affiliations. Both parties
recognize the importance of avoiding both actual conflicts and the appearance of
conflicts of interest. The parties will therefore mutually develop procedures
for identifying and evaluating actual, potential and apparent conflicts of
duality of interests. In discharging his duties and responsibilities hereunder,
Dr. Von Hoff will advise ILEX when and if an actual or potential conflict
arises. The parties will then mutually work at attempting to resolve the
conflict.

                  During the term of this Agreement, Consultant agrees to
provide services to ILEX as set forth in Exhibit B, and other activities as may
be requested by senior management of ILEX from time to time (collectively
"Services").

         2.       Compensation

                  A.       In exchange for Consultant's providing Services as
described above, during the term of this Agreement, ILEX shall pay Consultant
and Consultant agrees to accept

Page 1 of 12                                             ILEX(TM) Products, Inc.

<PAGE>

for Consultant's Services an annual fee of $275,000.00 USD paid at a monthly
rate of $22,916.66 by the 15th of each month.

                  B.       ILEX shall also reimburse Consultant for all
reasonable and necessary travel in accordance with ILEX's corporate policy and
will reimburse other reasonable and necessary out-of-pocket documented direct
expenses such as postage, facsimile, printing and long distance incurred in
connection with the Services. ILEX shall make these expense payments to
Consultant in accordance with this Agreement within thirty (30) days of receipt
of each detailed invoice with any receipts from Consultant indicating payment,
including travel as well as any reasonable and necessary incidental expenses
incurred in rendering such Services

                  C.       All payments, including reimbursements for actual
expenditures, shall be included in Consultant's gross income as compensation for
services rendered and accordingly reported on Consultant's IRS Form 1099.
Consultant is responsible for payment of all local, state and federal taxes
(including FICA taxes) on income earned under this Agreement, as none will be
withheld by ILEX. Consultant is not relying upon ILEX for advice regarding tax
aspects of this Agreement.

         3.       Independent Contractor

                  A.       It is agreed that Consultant is to have complete
freedom of action as to the details, methods, and means of performing the
consulting Services. It is further understood that Consultant has contracted
with ILEX only for the purposes and to the extent set forth in this Agreement
and Consultant's relation to ILEX is that of an independent contractor, and
Consultant shall be free to dispose of any portion of his entire time, energy,
and skill beyond that Consultant is obligated to devote to ILEX in such a manner
as Consultant sees fit and to such persons or entities as Consultant deems
advisable so long as any such other work by Consultant does not conflict with
the interest of ILEX.

                  B.       Consultant acknowledges that this Agreement creates a
relationship of special trust and confidence and imposes on him fiduciary duties
to act for the benefit of ILEX and its affiliated entities. By this Agreement,
ILEX retains Consultant only for the purposes and to the extent set forth
herein. As an independent contractor, Consultant does not have the authority to
bind ILEX or any of its affiliates to any third person, unless expressly agreed
to in writing by the President of ILEX.

                  C.       Neither Consultant nor Consultant's employees, agents
and/or subcontractors shall be considered, under the provisions of this
Agreement or otherwise, as having a status as an employee of ILEX nor shall
Consultant or his employees, agents and/or subcontractors be entitled hereafter
to participate in any plans, arrangements, or distributions by ILEX relating to
any pension, deferred compensation, bonus, stock bonus, hospitalization,

Page 2 of 12                                             ILEX(TM) Products, Inc.

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insurance, or other benefits extended to employees, since Consultant is
performing the consulting service hereunder as an independent contractor.

         4.       Term and Termination

                  A.       This Agreement shall come into being on its Effective
Date and will continue in effect until December 31, 2004 unless sooner
terminated. Either Consultant or ILEX may terminate this Agreement at any time
during its term by giving ninety (90) days notice of termination to the other
party.

                  B.       Sixty (60) days prior to the end of the term of this
Agreement, the parties shall enter into negotiations regarding the renewal of
this Agreement or the execution of a new agreement, which renewal or new
agreement shall contain such terms and conditions as may be agreed by the
parties.

                  C.       Regardless of whether this Agreement terminates at
the end of the agreed period above or as extended by supplemental agreement or
by any of the methods of early termination provided for herein, Consultant's
obligations set forth in Paragraphs 6, 7 and 8 below shall continue.

         5.       Representations and Warranties

         A.       Consultant represents and warrants that Consultant is free to
enter into this Agreement and perform the Services provided for in this
Agreement and will perform the Services in compliance with all applicable laws,
rules and regulations. Consultant agrees that all information Consultant
discloses to ILEX shall be received by ILEX without further obligation to
Consultant than as provided herein.

         B.       Consultant further represents that performance of all the
terms of this Agreement do not and will not breach any duties or obligations
owed under applicable law to third parties nor any agreement to keep in
confidence proprietary information, knowledge or data acquired in confidence or
in trust prior to the commencement of this Agreement and will not disclose to
ILEX or induce ILEX to use any confidential or proprietary information or
material belonging to any previous employer or others.

         C.       If Consultant is aware of circumstances under which ILEX may
not be free to use Consultant's information without liability of any kind, or
the use of which by ILEX would result in a possible infringement of one or more
unexpired patents or other proprietary rights known to Consultant, then
Consultant agrees to disclose such circumstances to ILEX and, upon disclosure of
such circumstance, Consultant shall be under no obligation to disclose further
related information to ILEX.

Page 3 of 12                                             ILEX(TM) Products, Inc.

<PAGE>

         6.       Ownership of Work Product

                  All work performed for ILEX by Consultant hereunder (including
but not limited to all data compiled by Consultant or reports, working notes,
drawings, design, and specifications developed or prepared by Consultant in
connection with such work) shall become the sole property of ILEX, unless
specifically otherwise agreed upon in writing by ILEX and Consultant. Consultant
hereby assigns to ILEX all rights related thereto, including copyright.

         7.       Non-Disclosure

         A.       Consultant shall protect, guard, and maintain the
confidentiality of Confidential Information (as defined in Paragraph 11), Except
as required in performance of Consultant's services for ILEX, Consultant will
never directly, indirectly, or otherwise, use, permit others to use,
disseminate, disclose, lecture upon, or publish articles concerning Confidential
Information, without ILEX's prior written consent. Further, Consultant agrees to
return Confidential Information supplied by ILEX and any copies thereof upon
completion of services relating to the Confidential Information or upon ILEX's
written request to do so. The obligations of confidentiality and non-use set
forth in this Agreement shall continue for a period often (10) years after
expiration or other termination of this Agreement.

         B.       Consultant may disclose Confidential Information to persons
within their direct employment who have a need to receive the Confidential
Information in order to further the purposes of this Agreement and who are bound
to protect the confidentiality of the Confidential Information. Consultant
agrees that all employees who are permitted access to Confidential Information
under this Agreement are bound to protect the confidentiality of such
information to the same extent as Consultant pursuant to this Agreement.

         C.       In the event that Consultant receives a subpoena, order, or
other process of any court or other governmental body seeking disclosure of
Confidential Information, Consultant shall notify ILEX immediately so that ILEX
shall have a reasonable amount of time prior to any court-ordered disclosure to
object to disclosure. Subject to the foregoing notice requirement, if required
by subpoena, order, or other process of any court or other governmental body,
Consultant may disclose the Confidential Information to a governmental authority
or by order of a court of competent jurisdiction, provided that such disclosure
is subject to all applicable governmental or judicial protection available for
like confidential or trade secret information.

         D.       Consultant hereby acknowledges that Consultant is aware that
the United States and other applicable securities laws prohibit any person who
has material, non-public information about a company from purchasing or selling
securities of such company or from communicating such information to any other
person under circumstances in which it is

Page 4 of 12                                             ILEX(TM) Products, Inc.

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reasonably foreseeable that such person is likely to purchase or sell such
securities. Consultant agrees not to disclose or use ILEX's Confidential
Information, directly or indirectly, to purchase, sell or trade, directly or
indirectly, securities of ILEX or of any other company which might be identified
in any Confidential Information, in violation of applicable law, including
Securities and Exchange Commission (SEC) Rules 10b5-l and 10b5-2.

         8.       Inventions, etc.

                  Consultant hereby assigns to ILEX all Inventions (as defined
in Paragraph 11) and any and all related patents, copyrights, trademarks, trade
names, and other industrial and intellectual property rights and applications
therefor, in the United States and elsewhere, and appoints any officer of ILEX
as his duly authorized agent to execute, file, prosecute and protect the same
before any government agency, court or authority. Consultant will ensure that
obligations under affiliations with any other persons or entities, including.,
without limitation, those persons or entities listed in EXHIBIT A, do not extend
to intellectual property rightfully owned by ILEX.

                  All Inventions (as defined in Paragraph 11) made, conceived or
completed by Consultant, individually or in conjunction with others, shall be
the sole and exclusive property of ILEX if said Inventions:

                  (a)      Are made with any of ILEX's equipment, supplies,
facilities or trade secrets; or

                  (b)      Are conceived or made by Consultant during the hours
Consultant is providing Services to ILEX; or

                  (c)      Relate to Confidential Information; or

                  (d)      Result from any work performed by Consultant for
ILEX.

                  Consultant shall, without royalty or any other further
consideration to Consultant therefore, but at the expense of ILEX:

                  (a)      As promptly as reasonably possible, disclose to ILEX
all information with respect to said inventions;

                  (b)      Whenever requested to do so by ILEX, promptly execute
and assign any and all applications, assignments and other instruments which
ILEX shall reasonably deem necessary to apply for and obtain Letters Patent of
the United States and of foreign countries for

Page 5 of 12                                             ILEX(TM) Products, Inc.

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said Inventions, and to assign and convey to ILEX or to ILEX's nominee the sole
exclusive rights, title and interest in and to the Inventions or any
applications or patents thereon;

                  (c)      Whenever requested to do so by ILEX deliver to ILEX
evidence for interference purposes or other legal proceedings and testify in any
interference or other legal proceedings; and

                  (d)      Do such other proper and lawful acts as may be
necessary in the opinion of ILEX to obtain and maintain United States and
foreign Letters Patent for the Inventions.

                  The foregoing provisions (as appropriate) shall also apply to
copyrights for copyrightable subject matter.

         9.       Survival of Certain Agreements

                  The covenants and agreements set forth in Paragraphs 6, 7, 8
and 9 shall survive termination or expiration of this Agreement and remain in
full force and effect.

         10.      Binding Effect

                  This Agreement shall be binding upon Consultant, and, except
as regards personal services, upon Consultant's heirs, personal representatives,
executors and administrators, and shall bind and inure to the benefit of ILEX,
its successors and assigns.

         11.      Definitions

                  As used in this Agreement:

                  (a)      "Confidential Information" means information
disclosed to Consultant or known by Consultant as a consequence of, or through
performance of services for, ILEX whether or not related to Consultant's duties
hereunder and includes trade secrets or any other like information of value
relating to the business and/or field of interest of any corporation, firm or
partnership directly or indirectly controlled by ILEX through more than five
percent (5%) ownership interest, including, but not limited to, information
relating to inventions, disclosures, processes, systems, methods, formulas,
patents, patent applications, machines, material, research activities and plans,
cost of production, contract forms, prices, volume of sales, promotional
methods, and lists of names or classes of customers. Information shall be
considered, for purposes of this Agreement, to be confidential if not known by
the trade generally, even though such information has been disclosed to one or
more third parties pursuant to any other agreement entered into by ILEX or any
of its affiliates.

Page 6 of 12                                             ILEX(TM) Products, Inc.

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                  Confidential Information shall not include information which:

                  (i)      was in the public domain at the time of disclosure,
as shown by ILEX's records; or

                  (ii)     after disclosure, becomes a part of the public domain
by publication or otherwise through no violation of this Agreement by Consultant
or violation of law or agreement by any other person;; or

                  (iii)    was, at the time of disclosure, in possession of
Consultant, otherwise than through, or in connection with, its services to ILEX
hereunder, and Consultant is able to so demonstrate possession of this
Confidential Information in writing to ILEX within sixty (60) days of receipt of
Confidential Information, or forego application of this provision; or

                  (iv)     is disclosed to Consultant without obligation of
confidence by a third party who did not acquire the same directly or indirectly
from ILEX under an obligation of secrecy.

                  Confidential Information disclosed or developed under this
Agreement shall not be deemed to be within the foregoing exceptions merely
because such information is embraced by more general information in the public
domain or in Consultant's possession.

(b)      "Inventions" means any and all discoveries, concepts and ideas, whether
patentable or not, including, but not limited to, processes, methods, formulas,
compositions, techniques, articles and machines, as well as improvements thereof
or know-how related thereto, relating to the business and/or field or interest
of ILEX or any corporation, firm or partnership directly or indirectly
controlled by ILEX or through more than a thirty percent (30%) ownership
interest.

         12.      Assignment

                  This Agreement shall not be assignable by the Consultant
without the prior written consent of ILEX; any attempted assignment is void.
ILEX may assign this Agreement to an affiliated company or parent company or
subsidiary or in connection with the merger, consolidation or sale of all or
substantially all of its assets.

         13.      Entire Agreement

                  This Agreement constitutes the entire agreement between the
parties, no representations having been made by any of the parties except as are
herein specifically set forth. No rights or obligations other than those
expressly recited herein are to be implied from this Agreement, It may be
amended only by an agreement in writing, signed by the party against

Page 7 of 12                                             ILEX(TM) Products, Inc.

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whom enforcement of any waiver, amendment modification, extension or discharge
is sought. Nothing herein shall require the parties to enter into further
agreements. This Agreement may be executed in any number of counterparts, each
of which shall be deemed an original and all of which shall constitute one and
the same instrument.

         14.      Waiver

                  A waiver or consent regarding any term, provision, or
condition of this Agreement given by ILEX on any one occasion shall be effective
only in that instance and shall not be construed as a bar or waiver of any right
on any other occasion.

         15.      Notices

                  Any correspondence or notifications concerning this Agreement
shall be deemed to have been duly given upon hand delivery or upon deposit in
the United States Mail, postage prepaid, certified or registered mail, return
receipt requested and will be addressed to ILEX and Consultant as follows:

                                   If to ILEX:

                               ILEX Products, Inc.
                     Attention: Jeffrey Buchalter, Director
                             4545 Horizon Hill Blvd.
                            San Antonio, Texas 78229
                                Fax: 210/949-8210

                                       and

                                If to Consultant:

                                    NDA, LLC
                       Attention: Daniel D. Von Hoff, M.D.
                           4951 N. Avenida de Vizcaya
                                Tucson, AZ 85718
                                Fax: 520/615-3726

or at some other address as shall have been furnished to the other in writing in
accordance herewith, except that such notice of such change shall be effective
only upon receipt.

         16.      Applicable Law

Page 8 of 12                                             ILEX(TM) Products, Inc.

<PAGE>

                  This Agreement shall be construed and performed in accordance
with the laws of the State of Texas, without (i) reference to the choice of law
principles thereof; or (ii) regard to any presumptions or rule requiring
construction or interpretation against the party drafting or causing the
Agreement to be drafted.

         17.      Miscellaneous

                  This Agreement is expressly conditioned upon obtaining the
review, consideration and approval of ILEX's Board of Directors.

         IN WITNESS WHEREOF, ILEX and Consultant have executed this Agreement
each on the date specified below by their duly authorized representatives.

ILEX PRODUCTS, INC.

By: /s/ Jeffrey H. Buchalter
    -------------------------------

Printed Name: Jeffrey H.  Buchalter               [STAMP OF APPROVED AS TO FORM]

Title: Director

Date: December 9, 2002

CONSULTANT

By: /s/ Daniel D. Von Hoff, M.D.
    ----------------------------
    Daniel D. Von Hoff, M.D.
    President of NDA, LLC

Date: December 10,2002

      74-2969148
-----------------------------------------
(Tax Identification/Social Security No.)

Page 9 of 12                                             ILEX(TM) Products, Inc.

<PAGE>
                                    EXHIBIT A

                          AFFILIATIONS OF DR. VON HOFF

BOARD MEMBERSHIPS (2002):

Genta

AFFILIATIONS AND CONSULTING AGREEMENTS OF DR. VON HOFF:

         Other Outside Activities of Daniel D. Von Hoff, M.D. (11/24/02)

<TABLE>
<CAPTION>
         ENTITY                                 ACTIVITY
------------------------        -----------------------------------------
<S>                             <C>
Genta                           Member of the Board of Directors

U.S. Oncology                   Chief Scientific Officer

PharmaMar                       Scientific Advisor to the Board

Boston Millenia Partners        Consulting on technology investments

eMed Securities                 Consulting on technology investments

Paw Partners                    Consulting on technology investments

Affymetrix                      Member of Clinical Advisory Committee

Celgene                         Member of Corporate Advisory Board

Rimononest Ltd.                 Member of Board of Advisors

Antigenics                      Member of Scientific Advisory Board (SAB)

Aton Pharmaceutical             Member of SAB

CanBas                          Member of SAB

Cell Pathways                   Member of SAB

Cyternex                        Member of SAB and holder of founder's shares

Emerging Med.Com                Member of SAB

Forsetti Bio Sc.                Member of SAB

Genetronics                     Member of SAB

Glycogenesis                    Member of SAB

ImaRX Therapy                   Member of SAB

Introgen                        Member of SAB

Light Sciences                  Member of SAB

Merck                           Member of SAB

Norcus                          Member of SAB

PharmaNet                       Member of SAB

Pintex                          Member of SAB

PrivaComp                       Member of SAB
</TABLE>

Page 10 of 12                                            ILEX(TM) Products, Inc.

<PAGE>

<TABLE>
<S>                             <C>
Takeda                          Member of SAB

TarGen                          Member of SAB

Topo Target A/S                 Member of SAB

Ventana                         Member of SAB

VidaCare                        Member of SAB and holder of founder's shares

Y.M. Bioscience                 Occasional consulting on molecules/trial design

Aphton Pharma/Cato Res.         Occasional consulting on molecules/trial design

Apoliz                          Occasional consulting on molecules/trial design

Biomeasure                      Occasional consulting on molecules/trial design

C.A.C.                          Occasional consulting on molecules/trial design

Centocor                        Occasional consulting on molecules/trial design

CombinatoRx                     Occasional consulting on molecules/trial design

Curis                           Occasional consulting on molecules/trial design

Geminz                          Occasional consulting on molecules/trial design

Gen Vec                         Occasional consulting on molecules/trial design

Glycodesign                     Occasional consulting on molecules/trial design

Human Genome Sciences           Occasional consulting on molecules/trial design

Medigene                        Occasional consulting on molecules/trial design

Merix Bioscience                Occasional consulting on molecules/trial design

Millenium Pharma.               Occasional consulting on molecules/trial design

Myriad Pharma.                  Occasional consulting on molecules/trial design

Oncolytics                      Occasional consulting on molecules/trial design

Peregrine Pharma.               Occasional consulting on molecules/trial design

Pfizer                          Occasional consulting on molecules/trial design

Research Corp. Tech.            Occasional consulting on molecules/trial design

Sang Stat                       Occasional consulting on molecules/trial design

Shire Pharma.                   Occasional consulting on molecules/trial design

Unither Pharma.                 Occasional consulting on molecules/trial design

Yamanouchi Pharma.              Occasional consulting on molecules/trial design
</TABLE>

Page 11 of 12                                            ILEX(TM) Products, Inc.

<PAGE>

                                    EXHIBIT B

Consultant shall perform the following services for ILEX personally by Dr. Von
Hoff and by no other employee or representative of NDA, LLC. Dr. Von Hoff shall
at all times exercise his independent judgment, discretion and control to
accomplish the stated objective.

(1)      Consultant will assist in the recruitment and hiring of a qualified
person to assume the role at ILEX as Chief Medical Officer.

(2)      Consultant shall physically attend Research & Development Management
Committee (RDMC) Meetings once a month at ILEX and at other times designated by
ILEX. Upon extenuating circumstances, Consultant may attend by conference call
with prior notice to Jeff Buchalter.

(3)      In conjunction with RDMC Meetings, Consultant shall spend an average of
two (2) days per month physically located at ILEX with one day a month as a
minimum

                  (a)      meeting with ILEX clinical development teams, and

                  (b)      reviewing New Drug Candidates (NDC) and protocols.

(4)      Consultant shall be available by phone to ILEX management and clinical
teams as needed.

(5)      At the request of ILEX, Consultant shall, consistent with the terms of
this Agreement, make presentations or publications for and on behalf of ILEX to
financial investors and/or potential financial investors regarding ILEX's
portfolio of compounds provided that such presentations or publication shall
include an appropriate disclosure of Consultant's financial relationship with
ILEX.

(6)      Consultant shall, consistent with the terms of this Agreement, advise
and assist the management of ILEX in establishing the strategic direction of
ILEX's clinical development plans.

(7)      Consultant shall assist ILEX in the identification and assessment of
potential licensing opportunities.

Page 12 of 12                                            ILEX(TM) Products, Inc.<PAGE>

                                  Exhibit 4.02

                          AMENDMENT TO RIGHTS AGREEMENT

         This is an Amendment to the Rights Agreement dated as of July 17, 1997
(the "Rights Agreement"), between Valero Energy Corp., formerly known as Valero
Refining and Marketing Co. (the "Company"), Harris Trust and Savings Bank
("Harris"), and Computershare Investor Services, LLC, ("Computershare").

                                   WITNESSETH

         WHEREAS, the Company and Harris previously entered into the Rights
Agreement, pursuant to which Harris was appointed to serve as the Rights Agent
under the Rights Agreement;

         WHEREAS, effective June 30, 2000, Computershare succeeded to the stock
transfer business of Harris, as a result of which , pursuant to Section 19 of
the Rights Agreement, Computershare became the Rights Agent for purposes of the
Rights Agreement;

         WHEREAS, the parties wish to amend the Rights Agreement to the extent
necessary to reflect such succession;

         WHEREAS, the Company and Computershare previously entered into
Amendment No. 1 to the Rights Agreement, pursuant to which the Company and
Computershare amended the Rights Agreement in contemplation of a merger between
the Company and UDS;

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

         Section 1. Amendment of Rights Agreement. Effective as of the date of
appointment of Computershare as successor Rights Agent, the Rights Agreement
shall be amended as follows:

         (a) Section 26 of the Rights Agreement is hereby amended by deleting
         the address for notice or demand to be given to the Rights Agent
         therein and substituting in lieu thereof the following:

                  "Computershare Investor Services, LLC
                  Two North LaSalle Street
                  Chicago, Illinois 60602
                  Attention:  Keith Bradley"

         (b) All references in the Rights Agreement to "Harris Trust and Savings
         Bank" as Rights Agent shall for all purposes be deemed to refer to
         "Computershare Investor Services, LLC."

                                       1
<PAGE>

         (c) Section 21 of the Rights Agreement is hereby amended by deleting
         the sentence that begins on page 36 with "Any successor Rights Agent. .
         . ." and ends on page 37 with ". . . combined capital and surplus of at
         least $50 million," and submitting in lieu thereof the following
         sentence:

                  Any successor Rights Agent, whether appointed by the Company
                  or by such a court, shall be (a) a corporation, limited
                  liability company or trust company (or similar form of entity
                  under the laws of any state of the United States or a foreign
                  jurisdiction) authorized to conduct business under the laws of
                  the United States or any state of the United States, which is
                  authorized under such laws to exercise corporate trust or
                  stockholder services powers and is subject to supervision or
                  examination by federal or state authority and which has at the
                  time of its appointment as Rights Agent a combined capital and
                  surplus of at least $10,000,000 or (b) an Affiliate controlled
                  by an entity described in clause (a) of this sentence.

         Section 3. Rights Agreement as Amended. The term "Agreement" as used in
the Rights Agreement shall be deemed to refer to the Rights Agreement as amended
hereby. This Amendment shall be effective as of June 30, 2000, and, except as
set forth herein, the Rights Agreement shall remain in full force and effect and
otherwise shall be unaffected hereby.

         Section 4. This Amendment shall be deemed to be a contract made under
the laws of the State of Delaware and for all purposes shall be governed by and
construed in accordance with the laws of such State applicable to contracts to
be made and performed entirely within such State.

         Section 5. This Amendment may be executed in any number of counterparts
and each of such counterparts shall for all purposes be deemed to be an
original, and all such counterparts shall together constitute but one and the
same instrument.

         Section 6. If any term, provision, covenant or restriction of this
Amendment is held by a court of competent jurisdiction or other authority to be
invalid, void or unenforceable, the remainder of the terms, provisions,
covenants and restrictions of this Amendment, and of the Rights Agreement, shall
remain in full force and effect and shall in no way be affected, impaired or
invalidated.

         Section 7. Except as otherwise expressly provided herein, or unless the
context otherwise requires, all terms used herein have the meanings assigned to
them in the Rights Agreement.

                                      * * *

                                       2
<PAGE>

         IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
duly executed as of the day and year above written.

                                          VALERO ENERGY CORP.

                                          By:      /s/ Jay D. Browning
                                             -----------------------------------
                                          Name:      Jay D. Browning
                                               ---------------------------------
                                          Title:   Vice President and Secretary
                                                --------------------------------

                                          HARRIS TRUST AND SAVINGS BANK

                                          By:      /s/ Martin J. McHale
                                             -----------------------------------
                                          Name:    Martin J. McHale
                                               ---------------------------------
                                          Title:   Vice President
                                                --------------------------------

                                          COMPUTERSHARE INVESTOR
                                          SERVICES, LLC

                                          By:      /s/ Keith A. Bradley
                                             -----------------------------------
                                          Name:    Keith A. Bradley
                                               ---------------------------------
                                          Title:   Director, Client Services
                                                --------------------------------

                                       3

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