Exhibit 5.1



                           [LETTERHEAD OF MCKESSON]


                               January 12, 1999


McKesson HBOC, Inc. 
McKesson Plaza
One Post Street
San Francisco, California 94104

     Re:  McKesson HBOC, Inc.  -- Registration Statement on Form S-8
          -----------------------------------------------------------

Ladies and Gentlemen:

I am Senior Vice President and General Counsel of McKesson HBOC, Inc., a
Delaware corporation (the "Company"), and am issuing this opinion in connection
with the Registration Statement on Form S-8 being filed by the Company with the
Securities and Exchange Commission (the "Commission") on the date hereof (the
"Registration Statement") for the purpose of registering with the Commission
under the Securities Act of 1933, as amended (the "Securities Act"), 10,597,085
shares (the "Shares") of common stock of the Company, par value $0.01 per share,
to be issued by the Company upon exercise of options to be granted or stock to
be sold pursuant to the stock option plans and employee stock purchase plans
(the "Plans") to be assumed pursuant to the Agreement and Plan of Merger, dated
as of October 17, 1998, as amended as of November 9, 1998 and as further amended
as of January 12, 1999 (the "Merger Agreement"), by and among McKesson
Corporation, McKesson Merger Sub, Inc., a Delaware corporation and a wholly
owned subsidiary of the Company ("Merger Sub"), and HBO & Company, a Delaware
corporation ("HBOC"), which Plans are set forth on the cover page of the
Registration Statement.

In this connection, I have reviewed the Registration Statement, as proposed to
be filed with the Commission.  As General Counsel, I am familiar with the
Restated Certificate of Incorporation of the Company, as amended, and the
Restated Bylaws of the Company, as amended, each as currently in effect.  I have
also examined originals or copies, certified or otherwise identified to my
satisfaction of such records of the Company and such instruments, certificates
of public officials, and such other documents, certificates and records as I
have deemed necessary or appropriate as a basis for the opinion set forth
herein.

In my examination, I have assumed the genuineness of all signatures, the legal
capacity of natural persons, the authenticity of all documents submitted to me
as originals, the conformity to original documents of all documents submitted to
me as certified, conformed or photostatic copies and the authenticity of the
originals of such copies.  In making my examination of documents executed or to
be executed by parties other than the Company, I have assumed that such parties
had or will 

 
have the power, corporate or other, to enter into and perform all obligations
thereunder and have also assumed the due authorization by all requisite action,
corporate or other, and execution and delivery by such parties and the validity
and binding effect thereof.  As to any facts material to the opinion expressed
herein which I have not independently established or verified, I have relied
upon statements and representations of other officers and representatives of the
Company and others.

I am admitted to the Bar of the State of California and do not purport to be an
expert on, or express any opinion concerning, any law other than the substantive
law of the State of California.

Based upon and subject to the foregoing, I am of the opinion that the Shares,
when issued and sold in accordance with the terms of the respective Plan to
which they relate, will be legally issued, fully paid and nonassessable.

I hereby consent to the filing of this opinion as an exhibit to the Registration
Statement.

This opinion is furnished by me, as counsel to the Company, in accordance with
the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act
and, except as provided in the immediately preceding paragraph, is not to be
used, circulated or quoted for any other purpose or otherwise referred to or
relied upon by any other person without the express written permission of the
Company.

                                       Very truly yours,

                                       /s/ Ivan D. Meyerson
                                       ----------------------------------
                                       Ivan D. Meyerson