SHUMAKER WILLIAMS, P.C.
                             3425 SIMPSON FERRY ROAD
                          CAMP HILL, PENNSYLVANIA 17108
                                 (717) 763-1121
   

                                           November 4, 1998


Board of Directors
ACNB Corporation
675 Old Harrisburg Road
Gettysburg, PA 17325

                  RE:      Registration Statement on Form S-4
                           Our File No. 742-98

Ladies/Gentlemen:

     In connection with the proposed  offering of up to 561,968 shares of common
stock, par value $2.50 per share (the "Common Stock"),  by ACNB Corporation (the
"Company"),  covered by the  Company's  Registration  Statement on Form S-4 (the
"Registration  Statement")  filed with the  Securities  and Exchange  Commission
under the Securities Act of 1933, as amended,  with respect to the Common Stock,
we, as special counsel to the Company, have reviewed:

     1.   Articles of Incorporation of the Company;

     2.   The Bylaws of the Company;

     3.   Resolutions  adopted by the Board of Directors of the Company relating
          to the  Registration  Statement,  certified  by the  Secretary  of the
          Company;

     4.   The Agreement and Plan of  Reorganization,  dated as of July 28, 1998,
          by and among, the Company, ACNB North, Inc., Farmers National Bancorp,
          Inc. and Farmers National Bank of Newville (the "Agreement");

     5.   The Registration Statement; and

     6.   Copies of the certificates representing shares of the Common Stock.
    
         Based on our review of the foregoing, it is our opinion that:

          a.   The  Company  has been  duly  incorporated  under the laws of the
               Commonwealth of Pennsylvania  and is validly existing and in good
               standing under the laws of the Commonwealth; and

          b.   The Common Stock covered by the  Registration  Statement has been
               duly  authorized and, when issued pursuant to the terms described
               in the Registration Statement and the Agreement,  will be legally
               issued by the Company and fully paid and non-assessable.

     We consent to the filing of this opinion as an exhibit to the  Registration
Statement and the reference to us in the related Proxy Statement/Prospectus.  In
giving this consent, we do not thereby admit that we come within the category of
persons whose consent is required under Section 7 of the Securities Act of 1933,
as  amended,  or the  rules  and  regulations  of the  Securities  and  Exchange
Commission thereunder.

                                             Very truly yours,



                                             /s/ Shumaker Williams, P.C.
                                             --------------------------
                                             SHUMAKER WILLIAMS, P.C.