The Collaborative International Dictionary of English v.0.48:

Dereplication \De`rep*li*ca"tion\, n. (Biochem.)
   the process of testing samples of mixtures which are active
   in a screening process, so as to recognize and eliminate from
   consideration those active substances already studied; -- a
   stage subsequent to the preliminary screening in the process
   of discovery of new pharmacologically active substances in
   mixtures of natural products; -- also called
   counterscreening. See screening.

   Note: In the process of pharmaceutical screening (testing a
         large number of substances to find those having
         desirable pharmacological activity), the testing of
         samples of substances extracted from living organisms
         (plants, microorganisms, etc.) often detects substances
         already detected in prior screening. Such "known" or
         "replicate" activities must be recognized at an early
         stage to avoid duplicating previous efforts at
         purification and structural identification. The process
         of testing an sample which is active in a primary
         screen, to determine if the activity is due to a
         previously known substance, is called dereplication or
         counterscreening.
         [PJC]