
<PAGE>

                                                                     EXHIBIT 4.3

THIS WARRANT AND THE UNDERLYING SHARES OF THE COMMON STOCK OF THE COMPANY TO BE 
ACQUIRED UPON THE EXERCISE HEREOF HAVE NOT BEEN REGISTERED UNDER THE SECURITIES 
ACT OF 1933, AS AMENDED, OR THE SECURITIES LAWS OF ANY STATE. SUCH SECURITIES 
MAY NOT BE SOLD, OFFERED FOR SALE, PLEDGED, ASSIGNED OR OTHERWISE TRANSFERRED 
EXCEPT UPON SUCH REGISTRATION OR UPON DELIVERY TO THE COMPANY OF AN OPINION OF 
COUNSEL SATISFACTORY TO THE COMPANY THAT REGISTRATION IS NOT REQUIRED FOR SUCH 
SALE, PLEDGE, ASSIGNMENT OR TRANSFER.

THE TRANSFER OF THIS WARRANT AND THE SHARES OF COMMON STOCK ISSUABLE UPON 
EXERCISE HEREOF IS SUBJECT TO COMPLIANCE WITH THE CONDITIONS SPECIFIED BELOW, 
AND NO TRANSFER OF THIS WARRANT OR SUCH SHARES SHALL BE VALID UNTIL SUCH 
CONDITIONS HAVE BEEN FULFILLED.

                        CLINICAL RESOURCE SYSTEMS, INC.

             COMMON STOCK PURCHASE WARRANT EXPIRING AUGUST 9, 1996

     This Is to Certify That, for value received, Dr. Robert G. McConnell (the 
"holder"), upon due exercise of this Warrant, is entitled to purchase from 
Clinical Resource Systems, Inc., a Texas corporation (the "Company"), before the
close of business on August 9, 1996 (the "Expiration Date"), 66,670 shares of 
fully paid and nonassessable common stock $.002 par value, of the Company (the 
"Common Stock"), at a purchase price of $1.50 per share (the "Initial Purchase 
Price").

     This Warrant is hereinafter called the "Warrant" and the shares of Common 
Stock issuable upon exercise hereof are hereinafter called the "Warrant Shares".

     Section 1.  Exercise of Warrant. The holder of this Warrant may exercise 
     -------------------------------
this Warrant for the purchase of the shares of Common Stock during such Exercise
Periods and for such Exercise Amounts as set forth below:

<TABLE> 
<CAPTION> 
           Exercise Periods               Exercise Amounts
           ----------------               ----------------
          <S>                            <C>
           8/9/91 - 8/8/92                13,334 shares not exercised
           8/9/92 - 8/8/93                13,334 shares 
           8/9/93 - 8/8/94                13,334 shares
           8/9/94 - 8/8/95                13,334 shares
           8/9/95 - 8/8/96                13,334 shares
</TABLE> 

     The above options to exercise this Warrant shall be referred to herein as 
the "Exercise Options". In order to exercise any of the Exercise Options of
this Warrant, the holder hereof shall deliver to the Company (i) a written
notice of such holder's election to exercise such Exercise Option of this
Warrant, (ii) payment of the aggregate purchase price of the shares of Common
Stock being purchased by certified or cashier's check and (iii) this Warrant;
provided that, in case such shares shall not have been registered under the
Securities Act of 1933 (the "Securities

<PAGE>
 
Act"), the Company may require that such holder furnish to the Company a written
statement that such holder is purchasing such shares for such holder's own
account for investment and not with a view to the distribution thereof, and that
none of such shares will be offered or sold in violation of the provisions of
the Securities Act or other applicable state securities law. Upon receipt
thereof, the Company shall, as promptly as practicable, execute or cause to be 
executed and deliver to such holder a certificate or certificates representing
the aggregate number of shares of Common Stock obtained pursuant to the exercise
of the Exercise Option of this Warrant. The stock certificate or certificates so
delivered shall be in such denominations as may be specified in said notice and
shall be registered in the name of such holder or, subject to Section 3, such
other name as shall be designated in said notice.

  No fractional shares of Common Stock are to be issued upon the exercise of
this Warrant, and the Company shall make no cash adjustment in respect of any
fraction of a share which would otherwise be issuable. The Company shall pay all
expenses, taxes and other charges payable in connection with the preparation,
execution and delivery of stock certificates under this Section, except that, in
case such stock certificates shall be registered in a name or names other than
the name of the holder of this Warrant, funds sufficient to pay all stock
transfer taxes which shall be payable upon the execution and delivery of such
stock certificates or certificates shall be paid by the holder hereof at the
time of delivering the notice of exercise mentioned above.

  The Company represents, warrants and agrees that all shares of Common Stock 
issuable upon any exercise of this Warrant shall be validly authorized and duly 
issued, fully paid and nonassessable.

  Prior to its exercise, this Warrant shall  not entitle the holder hereof to 
any of the rights of a shareholder of the Company.

  Section 2. Transfer, Division and Combination. This Warrant is transferable in
  ----------------------------------------------
the same manner and with the same effect as in the case of a negotiable 
instrument payable to a specified person. The Company, however, may treat the 
registered holder hereof as the owner hereof for all purposes until this Warrant
shall have been surrendered for transfer as hereinafter provided.

  Section 3. Restrictions on Transfer; Compliance with Securities Act.
  --------------------------------------------------------------------
  A. This Warrant and the related Warrant Shares shall not be transferable 
except upon the conditions specified in this Section, which conditions are 
intended, among other things to insure compliance with the provisions of the 
Securities Act in respect of the transfer of this Warrant or such Warrant 
Shares.

  B. The holder of this Warrant by acceptance hereof agrees, prior to any 
transfer or attempted transfer of this Warrant or the

                                       2
<PAGE>
 
related Warrant Shares, to give written notice to the Company of such holder's 
intention to effect such transfer. Each such notice (i) shall describe the 
manner and circumstances of the proposed transfer in sufficient detail and shall
contain an undertaking by the person giving such notice to furnish such other 
information as may be required to enable counsel to render the opinions referred
to below, and (ii) shall designate the counsel for the person giving such 
notice. The person giving such notice shall submit a copy thereof to the counsel
designated in such notice and the Company shall submit a copy thereof to its 
counsel, and the following provisions shall apply:

          (1)  If in the opinion of each such counsel the proposed transfer of 
     this Warrant or the Warrant Shares may be effected without registration of
     this Warrant or the Warrant Shares under the Securities Act or other
     applicable state securities laws, the Company shall, as promptly as
     practicable, so notify the holder of this Warrant or the Warrant Shares and
     such holder shall thereupon be entitled to transfer this Warrant or the
     Warrant Shares in accordance with the terms of the notice delivered by such
     holder to the Company.

          (2)  If in the opinion of either of such counsel the proposed transfer
     of this Warrant or the Warrant Shares may not be effected without
     registration of this Warrant or the Warrant Shares under the Securities Act
     or other applicable state securities laws, the Company shall, as promptly
     as practicable, so notify the holder of this Warrant or the Warrant Shares
     and the Company shall not be obligated to effect such transfer of this
     Warrant or the Warrant Shares, except pursuant to an offering made under a
     registration statement in accordance with applicable law.

     C.  The holder of this Warrant or the Warrant Shares hereby covenants and 
agrees with the Company as follows:

         (1)  The holder of this Warrant or Warrant Shares acknowledges being 
     informed that this Warrant or the Warrant Shares must be held by it
     indefinitely unless this Warrant or the Warrant Shares are registered for
     sale by such holder under the Securities Act and other applicable state
     securities laws or an exemption from such registration is available. Each
     such holder understands that any routine sale of the Warrant Shares made in
     reliance upon Rule 144 promulgated under the Securities Act can be made
     only in limited amounts after the expiration of a period of two years from
     the date of receipt of the Warrant Shares and otherwise in accordance with
     the terms and conditions of the Rule, and further understands that in the
     event that such Rule is not applicable, compliance with Regulation A or
     some other registration exemption under the Securities Act will be
     required.
<PAGE>
 
        (2) The Company may instruct its transfer agents not to transfer this 
     Warrant or the Warrant Shares unless such agents have been advised by the 
     Company or otherwise have been satisfied that such holder has complied 
     with the provisions above described.

        (3) Such holder understands that the Company has not covenanted and is 
     not obligated to furnish a registration statement under the Securities Act 
     covering the Warrant Shares, to file a notification under Regulation A 
     with respect to the Warrant Shares, or to take any other action that 
     would make available an exemption from registration.

        (4) This Warrant may not be transferred in part without the prior 
     consent of the Company, which consent may be granted or withheld in the 
     sole discretion of the Company.

     Section 4. Adjustment of Purchase Price.
     ----------------------------------------

     A. As used herein:

        (1) "Purchase Price" at any time shall mean the price per share of 
     Common Stock of the Company at which at such time this Warrant shall be 
     exercisable (including the Initial Purchase Price) in accordance with the 
     provisions hereof.

        (2) "Common Stock" shall mean stock of the Company of any class, whether
     now or hereafter authorized, which has the right to participate in the 
     distribution of either earnings or assets of the Company without limit 
     as to the amount or percentage; provided, however, that the term 
     "Warrant Shares" shall mean only the Common Stock, $.002 par value, of 
     the Company authorized at the date of this Warrant and stock of any other 
     class into which said presently authorized Common Stock may hereafter 
     have been changed. In case by reason of the operation of this Section 4 
     this Warrant shall be exercisable for any other shares of stock or other 
     securities or property of the Company or of any other corporation, any 
     reference herein to the exercise of this Warrant shall be deemed to refer 
     to and include the exercise of this Warrant for such other shares of 
     stock or other securities or property.

     B. The existence of this Warrant shall not affect in any way the right or 
power of the Company or its shareholders to make or authorize any or all 
adjustments, recapitalizations, reorganizations or other changes in the 
Company's capital structure or its business, or any merger or consolidation of 
the Company, or any issue of bonds, debentures, preferred or prior preference 
stock ahead of or affecting the Common Stock or the rights thereof, or the 
dissolution or liquidation of the Company, or any sale or transfer of all or any
part of its assets or business, or any other corporate act or proceeding, 
whether of a similar character or

                                       4
<PAGE>
 
otherwise. The Purchase Price shall be subject to adjustment from time to time 
as hereinafter provided (such price or such price as last adjusted pursuant to 
the terms hereof as the case may be, being herein called the "Purchase Price"). 
Upon each adjustment of the Purchase Price the holder of this Warrant shall  
thereafter be entitled upon exercise of this Warrant, at the adjusted Purchase
Price, the number of shares of Common Stock determined by (a) multiplying the 
Purchase Price in effect immediately prior to such adjustment by the number of 
Shares obtainable pursuant hereto immediately prior to such adjustment and (b) 
dividing the product thereof by the Purchase Price resulting from such 
adjustment.

          (1) If the Company shall issue or sell any shares of its Common Stock
     for a consideration per share less than the Purchase Price in effect
     immediately prior to the time of such issue or sale, then, forthwith upon
     such issue or sale, the Purchase Price shall be reduced to the price,
     calculated to the nearest cent, determined by dividing (i) an amount equal
     to the sum of (a) the number of shares of Common Stock outstanding
     immediately prior to such issue or sale multiplied by the then existing
     Purchase Price and (b) the consideration, if any, received by the Company
     upon such issue or sale by (ii) the total number of shares of Common Stock
     outstanding immediately after such issue or sale.

          (2) If the Company shall effect a subdivision or consolidation of
     shares or other capital readjustment, the payment of a stock dividend, or
     other increase or reduction of the number of shares of the Common Stock
     outstanding without receiving compensation therefor in money, services or
     property, then (a) the Purchase Price and number of shares of stock subject
     to this Warrant shall be appropriately adjusted in such a manner as to
     entitle the holder to receive upon exercise of this Warrant, for the same
     consideration, the same total number of shares as it would have received
     had it exercised this Warrant in full immediately prior to the event
     requiring the adjustment; and (b) the number of shares then reserved for
     issuance shall be adjusted by substituting for the total number and class
     of shares of stock then reserved that number of shares of stock that would
     have been received by the owner of an equal number of outstanding shares of
     Common Stock as the result of the event requiring the adjustment.

          (3) After a merger of one or more corporations into the Company, or
     after a consolidation of the Company and one or more corporations in which
     the Company shall be the surviving corporation, the holder of this Warrant
     shall, at no additional cost, be entitled upon exercise of this Warrant to
     receive in lieu of the number of shares as to which this Warrant shall then
     be so exercisable, the number of shares of stock or other securities or
     property to which such holder would have been entitled pursuant to the
     terms of the

                                       5
<PAGE>
 
     agreement of merger or consolidation if, immediately prior to such merger 
     or consolidation, such holder had been the holder of record of a number of 
     shares of Common Stock equal to the number and class of shares as to 
     which this Warrant shall be so exercised.

        (4) If the Company is merged into or consolidated with another 
     corporation under circumstances in which the Company is not the surviving 
     corporation, or if the Company is liquidated, or sells or otherwise 
     disposes of substantially all its assets to another corporation while 
     this Warrant remains outstanding after the effective date of such merger, 
     consolidation or sale, as the case may be, the holder of this Warrant 
     shall be entitled, upon exercise, to receive, in lieu of shares of Common 
     Stock, shares of such stock or other securities or property as the 
     holders of shares of Common Stock received pursuant to the terms of the 
     merger, consolidation or sale.

     Section 5. Special Agreements of the Company.
     ---------------------------------------------

     A. The Company covenants and agrees that it will reserve and set apart and 
have at all times a number of authorized but unissued shares of Common Stock 
deliverable upon the exercise of this Warrant or any other rights or privileges 
provided for herein sufficient to enable it at any time to fulfill all its 
obligations thereunder; if at any time the number of authorized but unissued 
shares of Common Stock shall not be sufficient to effect the exercise of this 
Warrant at the Purchase Price then in effect, the Company will take such 
corporation as may, in the opinion of its counsel, be necessary to increase its 
authorized but unissued shares of Common Stock to such number of shares as shall
be sufficient for such purposes.

     B. Whenever the number of shares purchasable under this Warrant or the 
Purchase Price shall be adjusted as required by the provisions of Section 4 
hereof, the Company shall forthwith mail a notice setting forth the adjusted 
Purchase Price and the adjusted number of Warrant Shares for which this Warrant 
is exercisable to the registered holder of this Warrant at his last address as 
it shall appear on the registration books, but failure to give or receive such 
notice, or any defects therein, or in the mailing thereof, shall not affect such
adjustment in Purchase Price.

     C. In case the Company proposes

        (1) to pay any stock dividend upon the Common Stock or make any 
     distribution (other than ordinary cash dividends payable out of earnings) 
     or offer any subscription or other rights to the holders of Common Stock, 
     or

        (2) to effect any capital reorganization or reclassification of capital 
     stock of the Company, or

                                       6
<PAGE>
 
          (3)  to effect the consolidation, merger, sale of all or substantially
     all of the assets, liquidation, dissolution or winding up of the Company,

     then the Company shall cause notice of any such intended action to be given
to the holder of this Warrant not less than 15 nor more than 30 days prior to
the date on which the transfer books of the Company shall close or a record be
taken for such dividend or distribution, or the date when such capital
reorganization, reclassification, consolidation, merger, sale, liquidation,
dissolution or winding up shall be effected, as the case may be.

     Section 6.  Notices. Any notice or other document required or permitted to
     -------------------
be given or delivered to the holder of this Warrant and the holders of the 
Warrant Shares shall be sent by certified or registered mail to each such holder
at the address shown on such holder's Warrant or such other address as shall 
have been furnished to the Company in writing by such holder. Any notice or 
other document required or permitted to be given or delivered to the Company 
shall be sent by certified or registered mail to the principal office of the 
Company at 1111 West 6th Street, Suite 230, Austin, Texas 78703, or such other 
address as shall have been furnished to the holders of this Warrant and the 
holders of Warrant Shares by the Company.

     Section 7.  Limitation of Liability. No provision hereof, in the absence of
     -----------------------------------
affirmative action by the holder to purchase shares of Common Stock, and no mere
enumeration herein of the rights or privileges of the holder hereof, shall give 
rise to any liability of such holder for the Purchase Price or as a shareholder 
of the Company, whether such liability is asserted by the Company or by
creditors of the Company.

     IN WITNESS WHEREOF, the Company has caused this Warrant to be signed in its
name by an authorized officer and accepted by the holder.


                                     CLINICAL RESOURCE SYSTEMS, INC.


                                     By: /s/ CHARLES B. OWEN, M.D.
                                         --------------------------
                                             Charles B. Owen, M.D.
                                             President

DATED: August 9, 1991


ACCEPTED on --------------


/s/ DR. ROBERT G. McCONNELL
- ---------------------------
    Dr. Robert G. McConnell

                                       7
<PAGE>
 
                                  ASSIGNMENT

                    TO BE EXECUTED BY THE REGISTERED HOLDER
                     IF HE DESIRES TO TRANSFER THE WARRANT

     FOR VALUE RECEIVED, __________________ hereby sells, assigns and 
transfers unto ________________________ the right to purchase ________ shares of
Common Stock, evidenced by the within Warrant, and does hereby irrevocably 
constitute and appoint __________________ Attorney to transfer the said Warrant 
on the books of the Company, with full power of substitution.


                                            ___________________________________
                                            Signature

Dated: _____________________________

In the presence of:                         ___________________________________

____________________________________        ___________________________________
                                            Address

     The Company hereby consents to the above assignment in part of ________ 
                                                         -------
shares of Common Stock.

                                        CLINICAL RESOURCE SYSTEMS, INC.

                                        By:____________________________________
                                        Name:__________________________________
                                        Title:_________________________________


                                    NOTICE

     The signature of the foregoing Assignment must correspond to the name as 
written upon the face of the within Warrant in every particular, without 
alteration or enlargement or any change whatsoever.
<PAGE>
 
                              NOTICE OF EXERCISE

                    TO BE EXECUTED BY THE REGISTERED HOLDER
                     IF HE DESIRES TO EXERCISE THE WARRANT

     The undersigned hereby exercises the right to purchase ________________ 
shares of stock covered by this Warrant according to the conditions thereof and 
herewith makes full payment of the Purchase Price of $________________ for such 
shares. Please issue a certificate representing said shares of Common Stock in 
the name of the undersigned.


                                            ___________________________________
                                            Signature

Dated: _____________________________        ___________________________________

                                            ___________________________________
                                            Address
