EXHIBIT 5.1

July 20, 2000


Marsh & McLennan Companies, Inc.
1166 Avenue of the Americas
New York, New York  10036-2774


Ladies and Gentlemen:


         I am General Counsel and Secretary of Marsh & McLennan Companies, Inc.,
a Delaware  corporation (the "Company").  I have acted as counsel to the Company
in connection  with the  preparation  and filing of a Registration  Statement on
Form S-8 (the  "Registration  Statement")  under the  Securities Act of 1933, as
amended,  (the "Securities  Act") for the  registration of 40,000,000  shares of
common stock,  par value $1.00 per share,  including  preferred  stock  purchase
rights (the "Shares"), of the Company relating to the Marsh & McLennan Companies
2000 Employee Incentive and Stock Award Plan (the "Plan").

         In connection with the foregoing,  I or attorneys under my supervision,
have examined the minute books and stock records of the Company, the Certificate
of Incorporation  and By-Laws of the Company,  the Registration  Statement,  the
Plan and  resolutions  duly  adopted by the Board of  Directors  of the  Company
relating to the Plan. In addition,  I, or attorneys under my  supervision,  have
reviewed such other  documents and  instruments  and have conferred with various
officers and  directors  of the Company and have  ascertained  or verified  such
additional  facts as deemed  necessary or  appropriate  for the purposes of this
opinion. In this examination I, or attorneys under my supervision,  have assumed
the legal capacity of all natural  persons,  the  genuineness of all signatures,
the  authenticity  of all documents  submitted as originals,  the  conformity to
original  documents of all  documents  submitted as  certified,  photostatic  or
facsimile copies and authenticity of the originals of such latter documents.

         Based  upon the  foregoing  I am of the  opinion  that the Shares to be
issued under the Plan have been duly  authorized  and, when issued and delivered
in accordance with the terms of the Plan, will be legally issued, fully paid and
nonassessable.

         My opinion is limited to matters  governed  by the laws of the State of
New York and the General Corporation Law of the State of Delaware.

         I hereby  consent  to the  filing of this  opinion as an exhibit to the
Registration  Statement.  In giving this  consent,  I do not hereby admit that I
come within the category of persons whose consent is required under Section 7 of
the Securities  Act or the rules and  regulations of the Securities and Exchange
Commission thereunder.

         This  opinion  is  solely  for  your  benefit  in  connection  with the
Company's  offer  and sale of the  Shares,  and is not to be  used,  circulated,
relied on,  quoted or  otherwise  referred to for any other  purpose  without my
express written permission.

                                                   Very truly yours,


                                                  /s/Gregory F. Van Gundy
                                                  Gregory F. Van Gundy
                                                  General Counsel