<PAGE>   1
                                                                   EXHIBIT 10.3


                         REGISTRATION RIGHTS AGREEMENT

         Registration Rights Agreement dated as of July 9, 1997 (this
"Agreement"), among ILEX ONCOLOGY, INC., a Delaware corporation (the
"Company"), and PRN Research, Inc., a Texas corporation ("PRN").

                             PRELIMINARY STATEMENT

         The Company and certain holders of its securities (the "Holders", or
if singular, a "Holder") are parties to the Fourth Amended and Restated
Registration Rights Agreement dated December 11, 1996 (the "Prior Agreement").
The Company and PRN are parties to a Service Agreement dated effective June 30,
1997 (the "Service Agreement") and an Assignment Agreement dated effective June
30, 1997 (the "Assignment Agreement"), pursuant to which PRN shall be issued
shares of Common (as defined herein). The Company desires to grant certain
registration rights to PRN in connection with such issuance of shares.
Therefore, in consideration of the mutual representations and agreements set
forth in this Agreement, the Company and PRN agree as follows:

                                   AGREEMENT

SECTION 1.       DEFINITIONS.

         As used in this Agreement, the following terms shall have the
following meanings:

         "Affiliate" means any entity controlling, controlled by or under
common control with a designated Person. For the purposes of this definition,
"control" shall have the meaning specified as of the date of this Agreement for
that word in Rule 405 promulgated by the Commission under the Securities Act.

         "Board" means the Board of Directors of the Company.

         "Commission" means the Securities and Exchange Commission, and any
successor thereto.

         "Common" means the Company's Common Stock, $.01 par value per share.

         "Exchange Act" means the Securities Exchange Act of 1934, as amended.

         "Immediate Family" shall mean spouses, descendants (including adopted
children) and spouses of descendants.

         "Person" means an individual, partnership, corporation, business
trust, limited liability company, joint stock company, trust, unincorporated
association, joint venture, or other entity of whatever nature.

         "Registrable Stock" means any shares of Common acquired by PRN
pursuant to the terms and provisions of the Service Agreement and the
Assignment





<PAGE>   2
Agreement and any and all shares of Common or other securities issued with
respect to such shares by way of stock splits, stock combinations and similar
events affecting the Registrable Stock. As to any particular Registrable Stock,
such securities will cease to be Registrable Stock when they shall have been
(x) effectively registered under the Securities Act and sold by the holder
thereof in accordance with such registration, or (y) sold to the public
pursuant to Rule 144 or Rule 701 of the Commission, or any successor rules.

         "Rule 144" means Rule 144 promulgated by the Commission under the
Securities Act, as such rule may be amended from time to time, or any successor
rule thereto.

         "Rule 701" means Rule 701 promulgated by the Commission under the
Securities Act, as such rule may be amended from time to time, or any successor
rule thereto.

         "Securities" means any debt or equity securities of the Company,
whether now or hereafter authorized, and any instrument convertible into, or
exercisable or exchangeable for, Securities or a Security.

         "Securities Act" means the Securities Act of 1933, as amended prior to
or after the date of this Agreement, or any federal statute or statutes which
shall be enacted to take the place of such Act, together with all rules and
regulations promulgated thereunder.

SECTION 2.       DEMAND REGISTRATIONS.

         2.1     PRN shall be entitled to five (5) demand registrations (a
"Demand Registration") between March 1, 1998, and September 15, 2003, by
providing a written notice to the Company requesting that the Company register
any eligible Registrable Stock specified in the notice, under the Securities
Act and under other relevant securities laws, for disposition in accordance
with methods stated in the notice.

         2.2     When it receives a registration notice under SECTION 2.1
above, the Company shall use its best efforts to effect the Demand Registration
under the Securities Act of Registrable Stock specified in the registration
notice under SECTION 2.1 to the extent requisite to permit disposition by PRN
in accordance with the intended methods of disposition described in the
registration notice.

         2.3     The Company shall use its best efforts to qualify for
registration on Form S-3 or any comparable or successor form or forms.

         SECTION 3.       INCIDENTAL/PIGGYBACK REGISTRATION. Each time the
Company proposes to register any of its Securities under the Securities Act for
its own account or for the account of other Security holders or both, it will
give at least 30 days advance written notice of its intention to do so to PRN.
PRN may then specify, by written notice to the Company within 25 days of the
date of the Company's notice, the number of shares of Registrable Stock held by
it that it wishes to include in the





                                     -2-
<PAGE>   3
Company's proposed registration (a "Piggyback Registration"). Subject to the
market cutback limitations of SECTION 8 of this Agreement, the Company will use
its best efforts to effect the Piggyback Registration under the Securities Act
of Registrable Stock specified by PRN under this SECTION 3.

         SECTION 4.       LIMITATIONS ON REGISTRATION RIGHTS. Notwithstanding
any contrary provision of this Agreement:

                 (a)      the Company shall not be required to effect (i) any
         Demand Registration that seeks to register and offer less than
         $500,000 aggregate amount of Registrable Stock, and (ii) per each
         twelve (12) month period, more than one Demand Registration; provided
         however, that a demand for a Demand Registration shall not count as a
         registration under this SECTION 4(A) if the registration statement
         filed with respect to such registration covering all shares of
         Registrable Stock specified in notices received as aforesaid (subject
         to the provisions of SECTION 8.1), for sale in accordance with the
         method of disposition specified by PRN, is not declared effective by
         the Commission and kept effective by the Company in accordance with
         the planned distribution thereunder, subject to SECTION 5.2, (unless,
         except as provided in SECTION 14(B), such Demand Registration has not
         become effective due solely to the fault of PRN and PRN fails to bear
         all Registration Expenses in connection therewith, in which case such
         registration shall count as a Demand Registration), and the last or
         any subsequent Demand Registration will not count as one of the
         permitted Demand Registrations unless PRN is able to register and sell
         at least 90% of the shares of the Registrable Stock requested to be
         included in such registration; and

                 (b)      The Company will not be obligated to effect any
         Demand Registration within ninety (90) days after the effective date
         of a Demand Registration or a registration in which PRN was given
         piggyback rights pursuant to SECTION 3, provided in either case all
         shares of Registrable Stock requested to be included were sold in such
         registration. The Company may postpone for up to ninety (90) days the
         filing or the effectiveness of a registration statement for a Demand
         Registration if the Company, in its sole discretion, determines that
         such registration would reasonably be expected to have an adverse
         effect (i) on any proposal or plan by the Company or any of its
         subsidiaries to engage in any acquisition of assets (other than in the
         ordinary course of business) or any merger, consolidation, tender
         offer or similar transaction, or (ii) any material corporate
         development; provided that in such event, PRN will be entitled to
         withdraw such request and, if such request is withdrawn, such
         registration will not count as one of the permitted Demand
         Registrations and the Company will pay all Registration Expenses (as
         defined in SECTION 6 hereof) in connection with such registration.

                 (c)      SECTION 3 of this Agreement shall not apply to a
         registration effected solely to offer securities for sale pursuant to,
         or in connection with, (i) an employee benefit plan or (ii) a
         transaction subject to Rule 145 under the Securities Act or in an
         exchange offer registered on Form S-4 or any successor





                                     -3-
<PAGE>   4
         form to Form S-4, or to any registration on a form which does not
         permit inclusion of Registrable Stock pursuant to Commission rule or
         practice.

SECTION 5.       REGISTRATION PROCEDURES.

                 5.1      Whenever the Company is required by this Agreement to
         use its best efforts to effect the registration of any Registrable
         Stock under the Securities Act, the Company will, as expeditiously as
         possible:

                 (a)      in the case of a Demand Registration, engage the
         underwriters as provided in SECTION 12;

                 (b)      before filing each registration statement or
         prospectus or amendment or supplement thereto with the Commission,
         furnish counsel for PRN with copies of all such documents proposed to
         be filed, which shall be subject to the reasonable approval of such
         counsel;

                 (c)      prepare and file with the Commission a registration
         statement with respect to such Registrable Stock (which, in the case
         of an underwritten public offering, shall be on Form S-1 or other form
         of general applicability satisfactory to the managing underwriter
         selected as therein provided) with respect to such securities
         including executing an undertaking to file post-effective amendments
         and use its best efforts to cause such registration statement to
         become and remain effective for the period of distribution
         contemplated thereby (subject to SECTION 5.2 below);

                 (d)      prepare and file with the Commission such amendments
         and supplements to such registration statement and the prospectus used
         in connection therewith as may be necessary to keep such registration
         statement effective for the period of distribution contemplated
         thereby (subject to SECTION 5.2 below) and to comply with the
         provisions of the Securities Act with respect to the sale or other
         disposition of all Registrable Stock covered by such registration
         statement in accordance with the sellers' intended methods of
         disposition set forth in such registration statement for such period;

                 (e)      promptly prepare and file with the Commission, and
         notify PRN immediately after the filing of, such amendment or
         supplement to such registration statement or prospectus as may be
         necessary to correct any statements or omissions if, during such
         periods as a prospectus relating to such Securities is required to be
         delivered under the Securities Act, any event shall have occurred as
         the result of which any such prospectus or any other prospectus as
         then in effect would include an untrue statement of a material fact or
         omit to state any material fact necessary to make the statements
         therein, in the light of the circumstances in which they were made,
         not misleading, and notify PRN and underwriter immediately after its
         discovery of such event;





                                     -4-
<PAGE>   5
                 (f)      furnish to the underwriters and PRN such numbers of
         copies of such registration statement, each amendment and supplement
         thereto (in each case, including all exhibits), the prospectus
         included in such registration statement (including each preliminary
         prospectus) and such other documents as such underwriters or PRN may
         reasonably request in order to facilitate the disposition of the
         Registrable Stock in accordance with such registration statement;

                 (g)      use its best efforts to register or qualify any
         Registrable Stock covered by such registration statement under the
         securities or blue sky laws of such jurisdictions within the United
         States of America as PRN or the underwriters reasonably request, and
         to take any other acts which a seller or the underwriters may
         reasonably request under such securities or blue sky laws to enable
         the consummation of the disposition in such jurisdictions of such
         Registrable Stock (provided, however, that the Company shall not be
         required under this Agreement (i) to qualify generally to do business
         as a foreign corporation in any jurisdiction in which it would not
         otherwise be required to qualify, or (ii) to consent to general
         service of process in any such jurisdiction unless the Company is
         already subject to service in such jurisdiction);

                 (h)      provide a transfer agent and registrar for all
         Registrable Stock sold under the registration statement not later than
         the effective date of the registration statement;

                 (i)      use its best efforts to cause all Registrable Stock
         sold under the registration statement to be listed on each securities
         exchange or to be qualified and eligible for trading in any automated
         quotation system, if any, on which similar Securities issued by the
         Company are then listed or traded or, if no such listing or
         qualification has then occurred, to use its best efforts to cause such
         Securities to be so listed or qualified on an exchange or in a trading
         system that is reasonably acceptable to the Company and the Holders of
         such Registrable Stock;

                 (j)      enter into such customary agreements (including
         underwriting agreements in customary form) and take all such other
         actions as the underwriters, if any, or PRN may reasonably request in
         order to expedite or facilitate the disposition of such Registrable
         Stock (including, without limitation, effecting a stock split or a
         combination of shares);

                 (k)      advise PRN, immediately after it shall receive notice
         or obtain knowledge thereof, of the issuance of any stop order by the
         Commission suspending the effectiveness of such registration statement
         or the initiation or threatening of any proceeding for such purpose
         and promptly use reasonable efforts to prevent the issuance of any
         stop order or to obtain its withdrawal if such stop order should be
         issued;

                 (l)      make available for inspection by PRN, any underwriter
         participating in any disposition pursuant to such registration
         statement, and





                                     -5-
<PAGE>   6
         any attorney, accountant or other agent retained by PRN or
         underwriter, all financial and other records, pertinent corporate
         documents and properties of the Company, and cause the Company's
         officers, directors, employees and independent accountants to supply
         all information reasonably requested by any such seller or underwriter
         in connection with such registration statement, all subject to such
         limitations as the Company reasonably deems appropriate in order to
         protect the Company's confidential or proprietary information;

                 (m)      comply with all applicable rules and regulations
         under the Securities Act and Exchange Act;

                 (n)      if the offering is underwritten and at the request of
         PRN, use its best efforts to furnish on the date that Registrable
         Stock are delivered to the underwriters for sale pursuant to such
         registration: (i) an opinion dated such date of counsel representing
         the Company for the purposes of such registration, addressed to the
         underwriters to such effects as reasonably may be requested by counsel
         for the underwriters and (ii) a letter dated such date from the
         independent public accountants retained by the Company, addressed to
         the underwriters stating that they are independent public accountants
         within the meaning of the Securities Act and that, in the opinion of
         such accountants, the financial statements of the Company included in
         the registration statement or the prospectus, or any amendment or
         supplement thereof, comply as to form in all material respects with
         the applicable accounting requirements of the Securities Act, and such
         letter shall additionally cover such other financial matters
         (including information as to the period ending no more than five (5)
         business days prior to the date of such letter) with respect to such
         registration as such underwriters reasonably may request;

                 (o)      make available for inspection by PRN, any underwriter
         participating in any distribution pursuant to such registration
         statement, and any attorney, accountant or other agent retained by PRN
         or underwriter, reasonable access to all financial and other records,
         pertinent corporate documents and properties of the Company, as such
         parties may reasonably request, and cause the Company's officers,
         directors and employees to supply all information reasonably requested
         by PRN, underwriter, attorney, accountant or agent in connection with
         such registration statement;

                 (p)      cooperate with PRN and the managing underwriter, if
         any, to facilitate the timely preparation and delivery of certificates
         representing Registrable Stock to be sold, such certificates to be in
         such denominations and registered in such names as PRN or the managing
         underwriter may request at least two (2) business days prior to any
         sale of Registrable Stock; and

                 (q)      permit PRN, if, in the sole and exclusive good faith
         judgment of the Company, PRN might be deemed to be a controlling
         person of the Company, to participate in good faith in the preparation
         of such registration statement.





                                     -6-
<PAGE>   7
         5.2     For purposes of this Agreement, the period of distribution of
Registrable Stock in a firm commitment underwritten public offering shall be
deemed to extend until each underwriter has completed the distribution of all
securities purchased by it, and the period of distribution of Registrable Stock
in any other registration shall be deemed to extend until the earlier of the
sale of all Registrable Stock covered thereby or 180 days after the effective
date thereof, provided, however, in the case of any registration of Registrable
Stock on Form S-3 or a comparable or successor form which are intended to be
offered on a continuous or delayed basis, such 180-day period shall be
extended, if necessary, to keep the registration statement effective until all
such Registrable Stock are sold, provided that Rule 415, or any successor rule
under the Securities Act, permits an offering on a continuous or delayed basis,
and provided further that applicable rules under the Securities Act governing
the obligation to file a post-effective amendment, permit, in lieu of filing a
post-effective amendment which (y) includes any prospectus required by Section
10(a)(3) of the Securities Act or (z) reflects facts or events representing a
material or fundamental change in the information set forth in the registration
statement, the incorporation by reference of information required to be
included in (y) and (z) above contained in periodic reports filed pursuant to
Section 13 or 15(d) of the Exchange Act in the registration statement.

         SECTION 6.       EXPENSES. The Company will pay all Registration
Expenses (as defined below) in connection with each Demand Registration or
Piggyback Registration of Registrable Stock permitted pursuant to SECTIONS 2
and 3 of this Agreement; provided, however, that the aggregate maximum
Registration Expenses for legal fees and disbursements, accounting fees and
disbursements and filing fees payable by the Company to third parties in
connection with all Demand Registrations shall be $100,000 and any such
expenses payable to third parties in excess of $100,000 shall be borne pro rata
by the selling stockholders and the Company. For purposes of this SECTION 6,
"Registration Expenses" means all expenses incident to the Company's
performance of or compliance with SECTIONS 2 and 3 of this Agreement,
including, without limitation, all registration, filing and National
Association of Securities Dealers, Inc. fees, all fees and expenses of
complying with securities or blue sky laws, all word processing, duplicating
and printing expenses, messenger and delivery expenses, the reasonable fees and
disbursements of counsel for the Company and of its independent public
accountants, including the expenses of any special audits or "cold comfort"
letters required by or incident to such performance and compliance, the
reasonable fees and disbursements of one law firm retained by the holders of
Registrable Stock being registered, premiums and other costs of policies of
insurance obtained by the Company against liabilities arising out of the public
offering of the Registrable Stock being registered but excluding all agency
fees and commissions, underwriting discounts and commissions and transfer
taxes, if any.

         SECTION 7.       INDEMNIFICATION.

         7.1     In the event of any registration of any of its Registrable
Stock under the Securities Act pursuant to this Agreement, the Company agrees,
to the extent permitted by law, to indemnify and hold harmless PRN, and each
officer, partner,





                                     -7-
<PAGE>   8
director and Affiliate of such seller, against any losses, claims, damages or
liabilities, joint or several, arising out of or based upon:

                 (a)      any untrue statement or alleged untrue statement of
         any material fact contained, on the effective date thereof, in any
         registration statement under which such Securities were registered
         under the Securities Act, any preliminary prospectus or final
         prospectus contained in any registration statement, or any other
         materials deemed to be a prospectus pursuant to the Securities Act, or
         any Securities being registered, or any amendment or supplement
         thereto, or any blue sky application or other document executed by the
         Company specifically for that purpose or based upon written
         information furnished by the Company filed in any state or other
         jurisdiction in order to qualify any or all of the Registrable Stock
         under the securities laws thereof (any such application, document or
         information herein called a "Blue Sky Application"), or

                 (b)      any omission or any alleged omission to state in any
         such document a material fact required to be stated therein or
         necessary to make the statements therein not misleading, or

                 (c)      any violation by the Company or its agents (other
         than by the indemnitee claiming indemnification hereunder) of the
         Securities Act or any rule or regulation promulgated under the
         Securities Act applicable to the Company or its agents (other than by
         the indemnitee claiming indemnification hereunder) and relating to
         action or inaction required of the Company in connection with such
         registration,

         except insofar as any such loss, claim, damage or liability is:

                          (i)     caused by or contained in any information
                 furnished in writing to the Company by PRN expressly for use
                 in connection with such registration; or

                          (ii)    caused by PRN's failure to deliver a copy of
                 the registration statement or prospectus or any amendment or
                 supplement thereto as required by the Securities Act or the
                 rules or regulations thereunder to be delivered by such
                 seller, if such delivery would have cured the defect giving
                 rise to such loss, claim, damage or liability; or

                          (iii)   caused by the delivery by PRN of a prospectus
                 or preliminary prospectus or any amendment or supplement
                 thereto after receipt of notice from the Company that it
                 should no longer be used.

         In connection with an underwritten offering, the Company will
         indemnify such underwriters, their officers and directors and each
         Person who controls (within the meaning of the Securities Act) such
         underwriters to the same extent as provided above with respect to PRN.
         The Company shall reimburse each Person indemnified pursuant to this
         SECTION 7.1 for any reasonable legal or other expenses incurred in
         connection with investigating or defending any





                                     -8-
<PAGE>   9
         loss, claim, damage, liability or action indemnified against. The
         reimbursements required by this SECTION 7.1 shall be made by periodic
         payments during the course of the investigation or defense, as and
         when bills are received or expenses incurred. The indemnities provided
         pursuant to this SECTION 7.1 shall remain in force and effect
         regardless of any investigation made by or on behalf of the
         indemnified party and shall survive any transfer of Registrable Stock
         by PRN.

         7.2     In the event of any registration of any Registrable Stock
under the Securities Act pursuant to this Agreement, PRN agrees to furnish to
the Company in writing such information and affidavits as the Company
reasonably requests for use in connection with any registration statement,
prospectus and any amendment or supplement thereto in connection with the
registration.

         7.3     To the fullest extent permitted by law, and subject to the
limitation set forth in the last sentence of this SECTION 7.3, PRN agrees to
indemnify and hold harmless the Company, its directors and officers, and each
Affiliate of the Company, against:

                 (a)      any losses, claims, damages or liabilities, joint or
         several, arising out of or based upon:

                          (i)     any alleged untrue statement of any material
                 fact contained on the effective date thereof, in any
                 registration statement under which such Securities were
                 registered under the Securities Act, any preliminary
                 prospectus or final prospectus contained therein, or any
                 summary prospectus contained therein, or any amendment or
                 supplement thereto, or

                          (ii)    any alleged omission to state in any such
                 document a material fact required to be stated therein or
                 necessary to make the statements therein not misleading,

         but only insofar as any such loss, claim, damage or liability is
         caused by any information furnished in writing to the Company by PRN
         expressly for use in connection with such registration, and excluding
         any such loss, claim, damage or liability which is caused by such
         statements, or caused by such omissions, based upon the authority of
         an expert as defined in the Securities Act (but only if PRN had no
         grounds to believe, and did not believe, that the statements made on
         the authority of an expert were untrue or that there was an omission
         to state a material fact); and

                 (b)      any losses, claims, damages or liabilities, joint or
         several, arising out of or based upon any failure by PRN to deliver a
         copy of the registration statement or prospectus or any amendment or
         supplement thereto if required by the Securities Act or the rules or
         regulations thereunder to be delivered by PRN, if such delivery would
         have cured the defect giving rise to such loss, claim, damage or
         liability. In connection with an underwritten offering, PRN will
         indemnify such underwriters, their officers and directors and each
         Person





                                     -9-
<PAGE>   10
         who controls (within the meaning of the Securities Act) such
         underwriters to the same extent as provided above with respect to the
         Company and other sellers. PRN shall reimburse each Person indemnified
         pursuant to this SECTION 7.3 in connection with investigating or
         defending any loss, claim, damage, liability or action indemnified
         against. The reimbursements required by this SECTION 7.3 shall be made
         by periodic payments during the course of the investigation or
         defense, as and when bills are received or expenses incurred. The
         indemnities provided pursuant to this SECTION 7.3 shall remain in
         force and effect regardless of any investigation made by or on behalf
         of the indemnified party and shall survive any transfer of Registrable
         Stock by PRN. Notwithstanding any contrary provision of this
         Agreement, however, the liability under this SECTION 7 of PRN shall be
         limited in the aggregate, with respect to the claims of all
         indemnified Persons taken as a whole, to the amount of proceeds
         received by PRN from the sale of the Registrable Stock sold by PRN
         pursuant to such registration statement.

         7.4     Each party entitled to indemnification under this SECTION 7
(the "Indemnified Party") shall give notice to the party required to provide
indemnification (the "Indemnifying Party") promptly after such Indemnified
Party has actual knowledge of any claim as to which indemnity may be sought,
and shall permit the Indemnifying Party to assume the defense of any such claim
or any litigation resulting therefrom; provided that counsel for the
Indemnifying Party, who shall conduct the defense of such claim or any
litigation resulting therefrom, shall be approved by the Indemnified Party
(whose approval shall not unreasonably be withheld, denied or delayed) and the
Indemnified Party may participate in such defense at such party's expense other
than reasonable costs of investigation and of liaison with counsel so selected
(unless the Indemnified Party shall have reasonably concluded that there may be
reasonable defenses available to it which are different from or additional to
those available to the Indemnifying Party or that the interests of the
Indemnified Party reasonably may be deemed to conflict with the interests of
the Indemnifying Party, in which case the Indemnified Party shall have the
right to select one separate counsel and to assume such legal defenses and
otherwise to participate in the defense of such action, with the reasonable
expenses and fees of such separate counsel and other reasonable expenses
related to such participation to be reimbursed by the Indemnifying Party as
incurred), and provided further that the failure of any Indemnified Party to
give notice as provided herein shall not relieve the Indemnifying Party of its
obligations under this SECTION 7 unless and to the extent the Indemnifying
Party is materially prejudiced thereby. No Indemnifying Party, in the defense
of any such claim or litigation, shall, except with the consent of each
Indemnified Party, consent to entry of any judgment or enter into any
settlement which does not include as an unconditional term thereof the giving
by the claimant or plaintiff to such Indemnified Party of a release from all
liability in respect to such claim or litigation. Each Indemnified Party shall
furnish such information regarding itself or the claim in question as an
Indemnifying Party may reasonably request in writing and as shall be reasonably
required in connection with the defense of such claim and litigation resulting
therefrom.

         7.5     If the indemnification provided for in this SECTION 7 is held
by a court of competent jurisdiction (by the entry of a final judgment or
decree by such court





                                    -10-
<PAGE>   11
and the expiration of time to appeal or the denial of the last right of appeal)
to be unavailable to an Indemnified Party with respect to any loss, liability,
claim, damage or expense referred to herein, then the Indemnifying Party, in
lieu of indemnifying such Indemnified Party hereunder, shall contribute to the
amount paid or payable by such Indemnified Party as a result of such loss,
liability, claim, damage or expense in such proportion as is appropriate to
reflect the relative fault of the Indemnifying Party on the one hand and of the
Indemnified Party on the other in connection with the statements or omissions
which resulted in such loss, liability, claim, damage or expense, as well as
any other relevant equitable considerations. The relative fault of the
Indemnifying Party and of the Indemnified Party shall be determined by
reference to, among other things, whether the untrue or alleged untrue
statement of a material fact or the omission to state a material fact relates
to information supplied by the Indemnifying Party or by the Indemnified Party
and the parties' relative intent, knowledge, access to information and
opportunity to correct or prevent such statement or omission. Notwithstanding
any contrary provision of this Agreement, however, the liability under this
SECTION 7.5 of each Holder which is a seller of Registrable Stock shall be
limited in the aggregate, with respect to the claims of all indemnified Persons
taken as a whole, to the amount of proceeds received by the indemnifying seller
from the sale of the Registrable Stock sold by the indemnifying seller pursuant
to such registration statement

         7.6     Notwithstanding the foregoing, to the extent that the
provisions on indemnification and contribution contained in the underwriting
agreement entered into in connection with any underwritten public offering
contemplated by this Agreement are in conflict with the foregoing provisions,
the provisions in such underwriting agreement shall be controlling. Not in
limitation of the foregoing, it is understood and agreed that the
indemnification obligations of PRN pursuant to any underwriting agreement
entered into in connection herewith shall be limited to the obligations
contained in this SECTION 7.

         7.7     The foregoing indemnity agreement of the Company and PRN is
subject to the condition that, insofar as they relate to any loss, claim,
liability or damage made in a preliminary prospectus but eliminated or remedied
in the amended prospectus on file with the Commission at the time the
registration statement in question becomes effective or the amended prospectus
filed with the Commission pursuant to Commission Rule 424(b) (the "Final
Prospectus"), such indemnity agreement shall not inure to the benefit of any
underwriter if a copy of the Final Prospectus was furnished to the underwriter
and was not sent or furnished to the person if required by law so to have been
delivered asserting the loss, liability, claim or damage at or prior to the
time such action is required by the Securities Act, and if the Final Prospectus
would have cured the defect giving rise to such loss, liability, claim or
damage.

         7.8     The indemnities and obligations provided in this SECTION 7
shall survive the transfer of any Registrable Stock by PRN.





                                                 -11-
<PAGE>   12
         SECTION 8.       MARKETING RESTRICTIONS.

         8.1     Notwithstanding any other provision of this Agreement, if:

                 (a)      two or more Holders of Common simultaneously seek to
         make a Demand Registration pursuant to a registration notice under
         SECTION 2 of this Agreement or under the Prior Agreement, and

                 (b)      the offering proposed to be made by the Holders of
         Common for whom such registration is to be made is to be an
         underwritten public offering, and

                 (c)      the managing underwriter or underwriters of such
         public offering furnish a written opinion to the Company and PRN that
         the total amount of Securities to be included in such offering would
         exceed the maximum number of shares of Common (as specified in such
         opinion) which can be marketed at a price reasonably related to the
         current market value of such Common and without otherwise materially
         and adversely affecting such offering (the "Underwriter Maximum"),

then the rights of all holders of Securities having the right to include Common
in such registration, to participate in such offering shall be in the following
order of priority:

                          (i)     FIRST, the number of shares of Investor
                 Registrable Stock (as such term is defined in the Prior
                 Agreement) requested to be included therein up to the
                 Underwriter Maximum allocated pro rata among the Holders of
                 such Investor Registrable Stock on the basis of the number of
                 shares of Investor Registrable Stock owned by such Holders
                 shall be entitled to participate, with further successive pro
                 rata allocations among the Holders of Investor Registrable
                 Stock if any such Holder of Investor Registrable Stock has
                 requested the registration of fewer than all of such shares of
                 Investor Registrable Stock that it is entitled to register;

                          (ii)    SECOND, the number of shares of CTRC
                 Registrable Stock (as such term is defined in the Prior
                 Agreement) requested to be included therein, up to the
                 Underwriter Maximum (after taking into account the number of
                 shares of Investor Registrable Stock to be sold pursuant to
                 (i) above) allocated pro rata among the Holders of such CTRC
                 Registrable Stock on the basis of the number of shares of CTRC
                 Registrable Stock owned by such Holders shall be entitled to
                 participate, with further successive pro rata allocations
                 among the Holders of CTRC Registrable Stock if any such Holder
                 of CTRC Registrable Stock has requested the registration of
                 fewer than all of such shares of CTRC Registrable Stock it is
                 entitled to register;

                          (iii)   THIRD, the number of shares of Management
                 Registrable Stock (as such term is defined in the Prior
                 Agreement) requested to be





                                    -12-
<PAGE>   13
                 included therein, up to the Underwriter Maximum (after taking
                 into account the number of shares of Investor Registrable Stock
                 and CTRC Registrable Stock to be sold pursuant to (i) and (ii)
                 above) allocated pro rata among the Holders of such Management
                 Registrable Stock on the basis of the number of shares of
                 Management Registrable Stock owned by such Holders shall be
                 entitled to participate, with further successive pro rata
                 allocations among the Holders of Management Registrable Stock
                 if any such Holder has requested the registration of fewer than
                 all of such shares of Management Registrable Stock it is
                 entitled to register;

                          (iv)    FOURTH, the number of shares of Registrable
                 Stock requested to be included therein, up to the Underwriter
                 Maximum (after taking into account the number of shares of
                 Investor Registrable Stock, CTRC Registrable Stock and
                 Management Registrable Stock to be sold pursuant to clauses
                 (i), (ii) and (iii) above) allocated pro rata among the
                 Holders of such Registrable Stock on the basis of the number
                 of shares of Registrable Stock owned by such Holders shall be
                 entitled to participate, with further successive pro rata
                 allocations among the Holders of Registrable Stock if any such
                 Holder has requested the registration of fewer than all of
                 such shares of Registrable Stock it is entitled to register;
                 and

                          (v)     FIFTH, other securities requested to be
                 included in such registration up to the Underwriter Maximum
                 (after taking into account the Investor Registrable Stock,
                 CTRC Registrable Stock, Management Registrable Stock and
                 Registrable Stock to be sold pursuant to clauses (i) through
                 (iv) above) shall be entitled to participate.

         8.2     If:

                 (a)      any Holder requests registration of Common under
         SECTION 3 of this Agreement, and

                 (b)      the offering proposed to be made is to be an
         underwritten public offering, and

                 (c)      the managing underwriters of such public offering
         furnish a written opinion that the total amount of Securities to be
         included in such offering would exceed the Underwriter Maximum,

then the rights of the Holders, of the holders of other Securities having the
right to include such Securities in such registration and of the Company to
participate in such offering shall be in the following order of priority:

                          (i)     FIRST, (1) if such registration was not
                 initiated by the Company as a primary registration, then the
                 Person or Persons requesting such registration pursuant to a
                 Demand Registration of such Person or Persons shall be
                 entitled to participate in accordance with





                                    -13-
<PAGE>   14
                 the relative priorities, if any, that shall exist among them,
                 subject to the limitation that a number of shares of Investor
                 Registrable Stock equal to 35% of the Underwriter Maximum
                 shall also be entitled to participate therein, with such
                 shares of Investor Registrable Stock being allocated pro rata
                 among the Holders of Investor Registrable Stock owned by such
                 Holders, and (2) if the Company has initiated such
                 registration as a primary registration, then the Company shall
                 be entitled to participate up to the full number of shares of
                 stock which the Company deems necessary or advisable to
                 fulfill its strategic capital requirements;

                          (ii)    SECOND, the number of shares of Investor
                 Registrable Stock requested to be included therein, up to the
                 remainder of such Underwriter Maximum (after taking into
                 account the number of shares to be sold pursuant to clause (i)
                 above) allocated pro rata among the Holders of such Investor
                 Registrable Stock on the basis of the number of shares of
                 Investor Registrable Stock owned by such Holders, with further
                 successive pro rata allocations among the Holders of Investor
                 Registrable Stock if any such Holder of Investor Registrable
                 Stock has requested the registration of fewer than all of such
                 shares of Investor Registrable Stock it is entitled to
                 register;

                          (iii)   THIRD, the number of shares of CTRC
                 Registrable Stock requested to be included therein, up to such
                 Underwriter Maximum (after taking into account the number of
                 shares to be sold pursuant to clauses (i) and (ii) above)
                 allocated pro rata among the Holders of such CTRC Registrable
                 Stock on the basis of the number of shares of CTRC Registrable
                 Stock owned by such Holders, with further successive pro rata
                 allocations among the Holders of CTRC Registrable Stock if any
                 such Holder of CTRC Registrable Stock has requested the
                 registration of fewer than all of such shares of CTRC
                 Registrable Stock it is entitled to register

                          (iv)    FOURTH, the number of shares of Management
                 Registrable Stock and Vector Registrable Stock (as such term
                 is defined in the Prior Agreement) requested to be included
                 therein, up to such Underwriter Maximum (after taking into
                 account the number of shares of securities to be sold pursuant
                 to clauses (i), (ii) and (iii) above) allocated pro rata among
                 the Holders of such Management Registrable Stock and Vector
                 Registrable Stock on the basis of the number of shares of
                 Management Registrable Stock and Vector Registrable Stock
                 owned by such Holders, with further successive pro rata
                 allocations among the Holders of Management Registrable Stock
                 and Vector Registrable Stock if any such Holder has requested
                 the registration of fewer than all of such shares of
                 Management Registrable Stock or Vector Registrable Stock he is
                 entitled to register;

                          (v)     FIFTH, the number of shares of Registrable
                 Stock requested to be included therein, up to the Underwriter
                 Maximum (after taking





                                    -14-
<PAGE>   15
                 into account the number of shares of securities to be sold
                 pursuant to clauses (i), (ii), (iii) and (iv) above) allocated
                 pro rata among the Holders of such Registrable Stock on the
                 basis of the number of shares of Registrable Stock owned by
                 such Holders shall be entitled to participate, with further
                 successive pro rata allocations among the Holders of
                 Registrable Stock if any such Holder has requested the
                 registration of fewer than all of such shares of Registrable
                 Stock it is entitled to register; and

                        (vi)      FIFTH, other securities requested to be
                 included in such registration up to the Underwriter Maximum
                 (after taking into account the securities to be sold pursuant
                 to clauses (i), (ii), (iii), (iv) and (v) above).

         8.3     In connection with any offering involving an underwriting of
Registrable Stock pursuant to SECTION 3 of this Agreement, the Company shall
not be required to include any of the Registrable Stock of a Holder in such
offering unless such Holder agrees to the terms of the underwriting agreed to
between the Company and the underwriter or underwriters selected by the
Company.

         SECTION 9.       LOCKUP AGREEMENT. PRN agrees in connection with the
registration of any Registrable Stock in a public offering that, upon the
request of the Company or the underwriters managing such underwritten offering
of the Company's Securities, it will not sell, make any short sale of, loan,
grant any option for the purchase of, or otherwise dispose of any Securities of
the Company (other than the securities included in the registration and other
than a transfer by PRN to an Affiliate of PRN) without the prior written
consent of the Company or such underwriters, as the case may be, for such
period of time (not to exceed 120 days) from the effective date of such
registration as the Company or the underwriters may specify, subject to the
Company obtaining similar agreements from the Company's executive officers and
directors, any holders of five (5) percent or more of the Company's then issued
and outstanding Common, and any other holders of Securities participating in
such registration.

         SECTION 10.      COMPLIANCE WITH RULE 144. With a view to making
available the benefits of certain rules and regulations of the Commission which
may permit the sale of restricted securities to the public without
registration, the Company agrees to:

                 (a)      make and keep public information available as those
         terms are understood and defined in Rule 144, at all times from and
         after ninety (90) days following the effective date of the first
         registration under the Securities Act filed by the Company for an
         offering of its Securities to the general public and for so long as
         the Company is subject to the reporting requirements of Section 13 or
         15(d) of the Exchange Act;

                 (b)      use its best efforts to file with the Commission in a
         timely manner all reports and other documents required of the Company
         under the





                                    -15-
<PAGE>   16
         Securities Act and the Exchange Act at any time after it has become
         subject to such reporting requirements; and

                 (c)      so long as PRN owns any Securities, and is not
         eligible to sell all such Securities under paragraph (k) of Rule 144,
         furnish to PRN upon request, a written statement by the Company as to
         its compliance with the reporting requirements of Rule 144 (at any
         time from and after ninety (90) days following the effective date of
         the first registration statement filed by the Company for an offering
         of its securities to the general public), and of the Securities Act
         and the Exchange Act (at any time after it has become subject to such
         reporting requirements), a copy of the most recent annual or quarterly
         report of the Company, and such other reports and documents so filed
         as PRN may reasonably request in availing itself of any rule or
         regulation of the Commission allowing PRN to sell any such securities
         without registration.

         SECTION 11.      ASSIGNABILITY OF REGISTRATION RIGHTS. The rights set
forth in this Agreement shall accrue to each subsequent holder of Registrable
Stock who (i) shall have executed a written consent agreeing to be bound by the
terms and conditions of this Agreement, and (ii) owns greater than 12,500
shares of Registrable Stock (subject to appropriate adjustment for stock
splits, stock combinations and similar events affecting the Registrable Stock).

         SECTION 12.      DESIGNATION OF UNDERWRITER. In the case of any Demand
Registration effected pursuant to this Agreement, the managing underwriter(s)
and any other investment banking advisers to the Company shall be selected by
the Company. The Company shall select the managing underwriter(s) and all other
investment banking advisers to the Company for all other registrations that may
be effected from time to time by the Company.

         SECTION 13.      HOLDBACK AGREEMENTS. The Company agrees (i) not to
effect any public sale or distribution of its equity securities, or any
securities convertible into or exchangeable or exercisable for such securities,
during the period commencing on the earlier of the notice requesting the Demand
Registration, if applicable or seven days prior to, and continuing during the
90-day period beginning on, the effective date of any underwritten Demand
Registration pursuant to SECTION 2 hereof or any underwritten Piggyback
Registration pursuant to SECTION 3 hereof (except as part of such underwritten
registration or pursuant to registrations on Form S-8 or any successor form),
unless the underwriters managing the registered public offering otherwise
agree, and (ii) to cause each holder of its Common, or any securities
convertible into or exchangeable or exercisable for Common, purchased from the
Company at any time after the date of this Agreement (other than in a
registered public offering) to agree not to effect any public sale or
distribution (including sales pursuant to Rule 144) of any such securities
during such period (except as part of such underwritten registration, if
otherwise permitted), unless the underwriters managing the registered public
offering otherwise agree.





                                    -16-
<PAGE>   17
         SECTION 14.      PARTICIPATION IN UNDERWRITTEN REGISTRATIONS.

                 (a)      No Person may participate in any registration
         hereunder which is underwritten unless such Person (i) agrees to sell
         such Person's securities on the basis provided in any underwriting
         arrangements approved by the Person or Persons entitled hereunder to
         approve such arrangements (including, without limitation, pursuant to
         the terms of any overallotment or "green shoe" option requested by the
         managing underwriter(s)), (ii) furnishes to the Company such
         information regarding such Person, the Registrable Stock of such
         Person to be registered and the intended method of disposition of such
         Registrable Stock, and (iii) completes and executes all
         questionnaires, powers of attorney, indemnities, underwriting
         agreements and other documents reasonably required under the terms of
         such underwriting arrangements.

                 (b)      If PRN disapproves of the terms of the underwriting,
         PRN may elect to withdraw therefrom by written notice to the Company
         and the managing underwriter; provided that if PRN withdraws, then
         subject to SECTION 4(A) hereof, (i) if such withdrawal occurs prior to
         the registration statement being filed with the Commission, PRN shall
         not be responsible for the Registration Expenses in connection with
         such registration, and (ii) if such withdrawal occurs subsequent to
         the filing of the registration statement with the Commission, then at
         the option of PRN, either PRN shall be responsible for the
         Registration Expenses incurred in connection with such registration,
         or such withdrawn registration shall count as a Demand Registration;
         provided, however, that if at the time of such withdrawal, PRN has
         learned of a material adverse change in the conditions, business or
         prospects of the Company from that known to them at the time of their
         request, then PRN shall not be required to pay any of such expenses or
         to count such registration as a Demand Registration.

                 (c)      Each Person that is participating in any registration
         hereunder agrees that, upon receipt of any notice from the Company of
         the happening of any event of the kind described in SECTION 5.1(E)
         above, such Person will forthwith discontinue the disposition of its
         Registrable Stock pursuant to the registration statement until such
         Person's receipt of the copies of a supplemented or amended prospectus
         as contemplated by such SECTION 5.1(E).

         SECTION 15.      MISCELLANEOUS.

         15.1    Modifications; Amendment. Neither this Agreement nor any
provision hereof may be changed, waived, discharged or terminated unless
effected by a writing executed and delivered by the Company and PRN.

         15.2    Severability. In the event that any court or any governmental
authority or agency declares all or any part of any Section of this Agreement
to be unlawful or invalid, such unlawfulness or invalidity shall not serve to
invalidate any other Section of this Agreement, and in the event that only a
portion of any Section is so declared to be unlawful or invalid, such
unlawfulness or invalidity shall not serve to invalidate the balance of such
Section.





                                    -17-
<PAGE>   18
         15.3    Successors and Assigns. Subject to SECTION 11, this Agreement
is binding upon and inures to the benefit of the Company, its successors and
assigns, and PRN, its successors and assigns, and legal representatives.

         15.4    Notices. All communications in connection with this Agreement
shall be in writing and shall be deemed properly given if hand delivered or
sent by telecopier or overnight courier with adequate evidence of delivery or
sent by registered or certified mail, return receipt requested, and, if to PRN,
at PRN's address as shown on the books of the Company or its transfer agent,
and if to the Company, at its offices at:

                                  ILEX ONCOLOGY, INC.
                                  11550 I.H. 10 West, Suite 300
                                  San Antonio, TX 78230
                                  Attention: President

Any notice called for hereunder shall be deemed given when received.

         15.5    Governing Law. This Agreement shall be governed by and
construed in accordance with the laws of the State of Texas applicable to
agreements between Texas residents entered into and to be performed entirely
within Texas.

         15.6    Counterparts. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original but all of which shall
together constitute one and the same instrument. A written consent executed
pursuant to SECTION 11 of this Agreement shall be deemed to be part of, and
constitute a counterpart of, this Agreement.

         15.7    Headings. The headings used herein are solely for the
convenience of the parties and shall not serve to modify or interpret the text
of the Sections at the beginning of which they appear.

         15.8    Entire Agreement. This Agreement embodies the entire agreement
and understanding among the Company and PRN and supersedes all prior oral and
written agreements and understandings relating to the subject matter hereof.

         15.9    Waiver. Any waiver or consent under this Agreement shall be
effective only in the specific instance and for the purpose for which it was
given, and shall not constitute a continuing waiver or consent except as
specifically set forth in such waiver or consent. The election of any remedy by
a party hereto shall not constitute a waiver of the right of such party to
pursue other available remedies. No notice to or demand on a party not
expressly required under this Agreement shall entitle the party receiving such
notice or demand to any other or further notice or demand in similar or other
circumstances or constitute a waiver of the rights of the party giving such
notice or demand to any other or further action in any circumstances without
such notice or demand.





                                    -18-
<PAGE>   19
         15.10   Specific Performance. The Company recognizes that the rights
of the PRN under this Agreement are unique and, accordingly, PRN shall, in
addition to such other remedies as may be available to them at law or in
equity, have the right to enforce their rights hereunder by actions for
injunctive relief and specific performance to the extent permitted by law.

         15.11   Grant of Registration Rights. The Company shall not hereafter
grant to any third party any registration rights more favorable than, or in any
way conflicting with, any of those contained herein, so long as any of the
registration rights under this Agreement remain in effect, provided, in any
event, (i) any grant of demand or required registration rights shall provide
that PRN has incidental or "piggyback" registration rights with respect thereto
in accordance with the provisions of SECTION 3 hereof, (ii) such rights shall
not become effective prior to the rights of PRN hereunder, and (iii) the
recipients of such rights shall be subject to provisions comparable to those
set forth in SECTION 9 hereof.



                         [signatures on following page]





                                    -19-
<PAGE>   20
         IN WITNESS WHEREOF, the parties hereto have caused this Agreement to
be executed on the day first above written.


The Company:
                                        ILEX ONCOLOGY, INC.

                                        By:
                                           ---------------------------------
                                        Name:
                                             -------------------------------
                                        Title:
                                              ------------------------------



PRN:
                                        PRN RESEARCH, INC.

                                        By:
                                           ---------------------------------
                                        Name:
                                             -------------------------------
                                        Title:
                                              ------------------------------





                                    -20-