Exhibit 5.1

                               January 16, 1998

Clark Refining & Marketing, Inc.
8182 Maryland Avenue
St. Louis, Missouri 63105

         Re:  Clark Refining & Marketing, Inc.
         Registration Statement on Form S-4
         ----------------------------------

Dear Ladies and Gentlemen:

    We have represented Clark Refining & Marketing, Inc., a Delaware corporation
(the "Company"), in connection with the preparation and filing with the
Securities and Exchange Commission under the Securities Act of 1933, as amended,
of a Registration Statement on Form S-4 (the "Registration Statement") relating
to the Company's 8 3/8% New Senior Notes due November 15, 2007 (the "New Senior
Notes") and 8 7/8% New Senior Subordinated Notes due November 15, 2007 (the "New
Senior Subordinated Notes" and, together with the New Senior Notes, the "New
Notes") to be issued under the indentures, dated as of November 21, 1997,
between the Company and Bankers Trust Company for the New Senior Notes, and
Marine Midland Bank for the New Senior Subordinated Notes.

          In connection with our representation, we have examined such corporate
and other records, instruments, certificates and documents as we consider
necessary to enable us to express the opinions set forth below.

          Based upon the foregoing, we are of the opinion that:

    1.  The Company is a corporation duly organized and validly existing in good
standing under the laws of the State of Delaware.

    2.  The Indentures constitute a valid and binding obligation of the Company
enforceable against the Company in accordance with its terms, subject to
applicable bankruptcy, insolvency, reorganization, moratorium and other laws
affecting the enforceability of creditors' rights generally and to court
decisions with respect thereto and to general principles of equity (regardless
of whether such enforceability is considered in a proceeding in equity or at
law).

          3. The New Notes, upon the due execution, authentication, issuance and
delivery thereof in accordance with the Indentures, will constitute valid and
legally binding obligations of the Company entitled to the benefits provided by
the Indentures, and enforceable in accordance with its terms, subject to
applicable bankruptcy, insolvency, reorganization, moratorium and other laws
affecting the enforceability of creditors' rights generally and to court
decisions with respect thereto and to general principles of equity (regardless
of whether such enforceability is considered in a proceeding in equity or at
law).

    We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to all references to this firm in such Registration
Statement.

                      Very truly yours,

                      /s/ Mayer, Brown & Platt

                      Mayer, Brown & Platt