We have hypothesized that ligand-induced binding sites (LIBS), i.e. sites expressed on cell surface receptors only after ligand binding causes the receptor to change shape, mediate subsequent biological events. To test this hypothesis, we have raised monoclonal antibodies that preferentially react with an integrin (platelet glycoprotein (GP) IIb-IIIa) after it bind Arg-Gly-Asp-containing ligands. The 13 anti-LIBS antibodies obtained define at least three distinct GPIIb-IIIa epitopes; one of these epitopes is also expressed following occupancy of another integrin, the vitronectin receptor. Certain of these LIBSs appear to mediate functions, since the antibodies that define them inhibit GPIIb-IIIa-mediated fibrin clot contraction or platelet adhesion to collagen. Nevertheless, none of the anti-LIBS antibodies inhibit binding of the primary ligand, fibrinogen. These data indicate that LIBS may mediate distinct consequences of receptor occupancy.