The Collaborative International Dictionary of English v.0.48:

Screening \Screen"ing\ (skr[=e]n"[i^]ng), n.
   the process of examining or testing objects methodically to
   find those having desirable properties. See screen[3].

   Note: In the pharmaceutical industry, pharmaceutical
         screening involves testing a large number of samples of
         substances to find those having desirable
         pharmacological activity; those samples which have the
         property sought are called active or positive in the
         screen. The substances tested may be pure compounds
         with known structure, mixtures of pure compounds, or
         complex mixtures obtained by extraction from living
         organisms. There are often additional sets of test
         performed on active samples, called

   counterscreening to eliminate those samples that may also
      possess undesirable properties. In the case of screening
      of mixtures from living organisms, a type of
      counterscreening called dereplication is usually
      performed, to determine if the active sample contains a
      known compound which has previously been studied.
      [PJC]

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]