
<PAGE>
 
                                                                     
                                                                  Exhibit 5     
                                 
                              March 29, 1999     
   
Case Corporation     
   
700 State Street     
   
Racine, Wisconsin 53404     
   
  Re: Registration Statement on Form S-3     
       
    $400,000,000 Principal Amount of Debt Securities     
   
Ladies and Gentlemen:     
   
  We have acted as counsel to Case Corporation, a Delaware corporation
("Case"), in connection with an offering pursuant to Rule 415 under the
Securities Act of 1933, as amended (the "Securities Act"), of $400 million
principal amount of Case's debt securities (the "Debt Securities") and the
corporate proceedings (the "Corporate Proceedings") taken and to be taken in
connection therewith. The Debt Securities are to be issued under an indenture
(the "Indenture"), between Case and The Bank of New York, as Trustee, dated as
of July 31, 1995, with certain terms of the Debt Securities to be established
by certain officers of Case who have been authorized by its Board of Directors
to do so, as part of the Corporate Proceedings relating to the issuance of the
Debt Securities.     
   
  We have also participated in the preparation and filing with the Securities
and Exchange Commission under the Securities Act of a Registration Statement on
Form S-3 (the "Registration Statement") relating to the Debt Securities. In
this connection, we have examined such corporate and other records,
instruments, certificates and documents as we considered necessary to enable us
to express this opinion.     
   
  Based on the foregoing, it is our opinion that the Indenture is a valid and
binding instrument and that, upon completion of the Corporate Proceedings, the
Debt Securities will have been duly authorized for issuance and, when each
series of Debt Securities is duly executed, authenticated, issued and
delivered, such series will constitute valid and legally binding obligations of
Case under the laws of the State of New York, entitled to the benefits of the
Indenture, subject to bankruptcy, insolvency, reorganization, moratorium and
similar laws of general applicability relating to or affecting creditor's
rights and to general equity principles (whether considered in a proceeding at
law or in equity).     
   
  We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to being named in the related prospectus and any
related prospectus supplement under the caption "Legal Matters" with respect to
the matters stated therein. Further, we hereby consent to the incorporation by
reference of this opinion and consent in any abbreviated registration
statement(s) registering up to an additional 20% aggregate principal amount of
Debt Securities filed subsequent to the date hereof.     
   
  We are admitted to practice law in the States of Illinois and New York, and
we express no opinions as to matters under or involving any laws other than the
laws of the States of Illinois and New York, the federal laws of the United
States of America and the General Corporation Law of the State of Delaware.
                                             
                                          Very truly yours,     
                                             
                                          Mayer, Brown & Platt     
