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                                                                     EXHIBIT 4.3

                                 FIRST AMENDMENT

         This First Amendment dated as of April 30, 1997 (the "First
Amendment"), between ENERGY BIOSYSTEMS (the "Company") and KEYCORP SHAREHOLDER
SERVICES, INC., successor in interest to Society National Bank (the "Agent").

                                   WITNESSETH

         WHEREAS, the Company and the Agent entered into that Stockholders
Rights Agreement dated as of March 8, 1995 (the "Agreement"); and

         WHEREAS, the Company and the Agent desire to amend the Agreement to
reflect a change in the requirements regarding the appointment of a successor
Agent.

         NOW, THEREFORE, in consideration of the mutual promises and other good
and valuable consideration, the receipt and sufficiency of which is hereby
acknowledged, the parties agree as follows:

         Section 1.01 DEFINED TERMS. Capitalized terms used in this First
Amendment and not defined shall have the meanings assigned to them in the
Agreement.

         Section 1.02 AMENDMENT OF SECTION 21. Section 21 of the Agreement shall
be amended as follows:

                  Section 21. CHANGE OF RIGHTS AGENT. The Rights Agent or any
         successor Rights Agent may resign and be discharged from its duties
         under this Agreement upon thirty (30) days' notice in writing mailed to
         the Company and to each transfer agent of the Preferred Shares and
         Common Shares by registered or certified mail, and to the holders of
         the Rights Certificates by first-class mail. The Company may remove the
         Rights Agent or any successor Rights Agent upon thirty (30) days'
         notice in writing, mailed to the Rights Agent or successor Rights
         Agent, as the case may be, and to each transfer agent of the Preferred
         Shares or Common Shares by registered or certified mail, and to the
         holders of the Rights Certificates by first-class mail. If the Rights
         Agent shall resign or be removed or shall otherwise become incapable of
         acting, the resigning, removed, or incapacitated Rights Agent shall
         remit to the Company, or to any successor Rights Agent designated by
         the Company, all books, records, funds, certificates or other documents
         or instruments of any kind then in its possession which were acquired
         by such resigning, removed or incapacitated Rights Agent in connection
         with its services as Rights Agent hereunder, and shall thereafter be
         discharged from all duties and obligations hereunder. Following notice
         of such removal, resignation or incapacity, the Company shall appoint a
         successor to such Rights Agent. If the Company shall fail to make such
         appointment within a period of thirty (30) days after giving notice of
         such removal or after it has been notified in writing of such
         resignation or incapacity by the resigning or incapacitated Rights
         Agent or by the holder of a Rights Certificate (who shall, with such
         notice, submit his Rights Certificate for inspection by the Company),
         then any registered holder of any Rights Certificate may apply to any
         court of competent jurisdiction for the appointment of a new Rights
         Agent. Any successor Rights Agent, whether appointed by the Company or
         by such a court, shall be either (a) a corporation organized and doing
         business under the laws of the United States, or of the State of New
         York, Texas, or Illinois (or of any other state of the United States so
         long as such corporation is authorized to do business as a banking
         institution in the State of New York or Texas), in good standing,
         having an office or an affiliate with an office in the State of New
         York, which is authorized under such laws to exercise corporate trust
         or stock transfer 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
         $50 million or (b) an affiliate of a corporation described in clause
         (a) of this sentence which is a corporation organized and doing
         business under the laws of the United States or of the State of New
         York or Texas (or of any other state of the United States so long as
         such corporation is authorized to do business as a banking institution
         in the State of New York or Texas), in good standing, having a
         principal office in the State of New York, which is authorized under
         such laws to exercise corporate trust or stock transfer powers and is
         subject to supervision or examination by federal or state authority.
         After appointment, the successor Rights Agent shall be vested

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         with the same powers, rights, duties and responsibilities as if it had
         been originally named as Rights Agent without further act or deed; but
         the predecessor Rights Agent shall deliver and transfer to the
         successor Rights Agent any property at the time held by it hereunder,
         and execute and deliver any further assurance, conveyance, act or deed
         necessary for the purpose. Not later than the effective date of any
         such appointment, the Company shall file notice thereof in writing with
         the predecessor Rights Agent and each transfer agent of the Common
         Shares or Preferred Shares, and mail a notice thereof in writing to the
         registered holders of the Rights Certificates. Failure to give any
         notice provided for in this Section 21, however, or any defect therein,
         shall not affect the legality or validity of the resignation or removal
         of the Rights Agent or the appointment of the successor Rights Agent,
         as the case may be.

         Section 1.03 AGREEMENT TO REMAIN IN FULL FORCE AND EFFECT. Other than
as specifically amended by this First Amendment, the other terms of the
Agreement shall remain in full force and effect.

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

                                       ENERGY BIOSYSTEMS CORPORATION

                                       By:  /s/ Paul G. Brown, III
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                                       KEYCORP SHAREHOLDER SERVICES, INC.

                                       By:  /s/ Barbara Shepherd
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