Abstract: The present invention relates to screening methods and, in particular, to methods of screening anti-ligands libraries for identifying anti-ligands specific for differentially and/or infrequently expressed ligands. The method comprises the steps of providing a library of anti-ligands; providing a first subtractor ligand; providing a second target ligand; determining the amount of the first and second target ligands using one or more equations derived from the universal law of mass action; providing the determined amount of a first subtractor ligand; providing the determined amount of a second target ligand; providing separation means capable of use to isolate anti-ligand bound to the second target ligand from anti-ligand bound to the first subtractor ligand; exposing the library of to the first and second target ligands to permit binding of anti-ligands to ligands; and using the separation means to isolate the anti-ligand bound to second target ligand.