EXHIBIT 99.5
                             McKESSON CORPORATION
 
                          OFFERS FOR ALL OUTSTANDING
                       6.30% NOTES DUE MARCH 1, 2005 AND
                         6.40% NOTES DUE MARCH 1, 2008
                                IN EXCHANGE FOR
                  6.30% EXCHANGE NOTES DUE MARCH 1, 2005 AND
                    6.40% EXCHANGE NOTES DUE MARCH 1, 2008
 
TO OUR CLIENTS:
 
  Enclosed for your consideration is a Prospectus, dated      , 1998 (the
"Prospectus"), and the related Letters of Transmittal (the "Letters of
Transmittal"), relating to the offers (the "Exchange Offers") of McKesson
Corporation (the "Company") to exchange its 6.30% Exchange Notes due March 1,
2005 and 6.40% Exchange Notes due March 1, 2008, which have been registered
under the Securities Act of 1933, as amended (the "Exchange Notes"), for its
outstanding 6.30% Notes due March 1, 2005 and 6.40% Notes due March 1, 2008
(the "Private Notes"), respectively, upon the terms and subject to the
conditions described in the Prospectus and the applicable Letters of
Transmittal. The Exchange Offers are being made in order to satisfy certain
obligations of the Company contained in the Registration Rights Agreement
dated February 24, 1998, by and among the Company and the initial purchasers
referred to therein.
 
  This material is being forwarded to you as the beneficial owner of the
Private Notes carried by us in your account but not registered in your name. A
TENDER OF SUCH PRIVATE NOTES MAY ONLY BE MADE BY US AS THE HOLDER OF RECORD
AND PURSUANT TO YOUR INSTRUCTIONS.
 
  Accordingly, we request instructions as to whether you wish us to tender on
your behalf any or all of the Private Notes held by us for your account,
pursuant to the terms and conditions set forth in the enclosed Prospectus and
applicable Letters of Transmittal.
 
  Your instructions should be forwarded to us as promptly as possible in order
to permit us to tender your Private Notes on your behalf in accordance with
the provisions of the Exchange Offers. The Exchange Offers will expire at 5:00
p.m., New York City time, on     , 1998, unless any of the Exchange Offers are
extended by the Company (the "Expiration Dates"). Any Private Notes tendered
pursuant to the Exchange Offers may be withdrawn at any time before the
applicable Expiration Date.
 
  Your attention is directed to the following:
 
  1. The Exchange Offers are for any and all Private Notes.
 
  2. The Exchange Offers are subject to certain conditions set forth in the
Prospectus in the section captioned "The Exchange Offers--Certain Conditions
to the Exchange Offers."
 
  3. Any transfer taxes incident to the transfer of Private Notes from the
holder to the Company will be paid by the Company, except as otherwise
provided in the Instructions in the applicable Letters of Transmittal.
 
  4. The Exchange Offers expire at 5:00 p.m., New York City time, on      ,
1998, unless any of the Exchange Offers are extended by the Company.
 
  If you wish to have us tender any or all of your Private Notes, please so
instruct us by completing, executing and returning to us the instruction form
on the back of this letter. THE LETTERS OF TRANSMITTAL ARE FURNISHED TO YOU
FOR INFORMATION ONLY AND MAY NOT BE USED DIRECTLY BY YOU TO TENDER YOUR
PRIVATE NOTES.

 
                         INSTRUCTIONS WITH RESPECT TO
                              THE EXCHANGE OFFERS
 
  The undersigned acknowledge(s) receipt of your letter and the enclosed
material referred to therein relating to the Exchange Offers made by McKesson
Corporation with respect to its Private Notes.
 
  This will instruct you to tender the Private Notes held by you for the
account of the undersigned, upon and subject to the terms and conditions set
forth in the Prospectus and the related applicable Letters of Transmittal.
 
  Please tender the Private Notes held by you for my account as indicated
below:
 
                                            Aggregate Principal Amount of
                                            Private Notes
 
6.30% Notes due March 1, 2005
                                            -------------------------------
 
6.40% Notes due March 1, 2008
                                            -------------------------------
 
[_] Please do not tender any Private Notes held
by you for my account
 
Dated:               , 1998
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                                                       Signature(s)
 
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                                               Please print name(s) here
 
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                                                        Address(es)
 
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                                            Area Code and Telephone Number
 
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                                            Tax Identification or Social
                                            Security No(s).
 
  None of the Private Notes held by us for your account will be tendered
unless we receive written instructions from you to do so. Unless a specific
contrary instruction is given in the space provided, your signature(s) hereon
shall constitute an instruction to us to tender all the Private Notes held by
us for your account.