Exhibit 5.1

                         Automatic Data Processing, Inc.
                             Corporate Headquarters
                                One ADP Boulevard
                         Roseland, New Jersey 07068-0456

                                                                  August 6, 1999

Board of Directors
Automatic Data Processing, Inc.
One ADP Boulevard
Roseland, New Jersey 07068-0456

                  Re:      Automatic Data Processing, Inc.
                           Registration Statement on Form S-8

Gentlemen:

         I have acted as counsel to Automatic Data Processing, Inc., a Delaware
corporation (the "Company"), in connection with the registration by the Company
of 361,488 shares of the Company's Common Stock, par value $0.10 per share (the
"Shares"), pursuant to the Company's Registration Statement on Form S-8 which is
to be filed with the Securities and Exchange Commission on August 6, 1999 (the
"Registration Statement").

         In this connection, I have examined originals or copies, certified or
otherwise identified to my satisfaction, of such corporate records, certificates
and written and oral statements of officers and accountants of the Company and
of public officials, and other


documents that I have considered necessary and appropriate for this opinion and,
based thereon, I advise you that, in my opinion:

         1.       The Company has been duly incorporated and is validly existing
                  under the laws of the State of Delaware.

         2.       The Company has corporate authority to issue the Shares in the
                  manner and under the terms set forth in the Registration
                  Statement.

         3.       The Shares have been duly authorized and, when issued in
                  accordance with (a) the OMR Systems Corporation Stock Option
                  Plan and (b) the Merger Agreement, each referred to in the
                  Registration Statement, will be validly issued, fully paid and
                  nonassessable.

         I hereby consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement, to its use as part of the Registration Statement, and to
the use of my name in the Registration Statement.

                                         Very truly yours,


                                         /s/ James B. Benson
                                         -------------------
                                         James B. Benson
                                         General Counsel