This study explores conformational states of human platelet glycoprotein IIIa (GP IIIa) and possible mechanisms of fibrinogen receptor exposure. D3GP3 is an IgG1, kappa monoclonal antibody generated against purified GP IIIa and found to be specific for GP IIIa by immunoprecipitation and Western blot analysis. The binding of D3GP3 to resting platelets caused fibrinogen binding (approximately 5,000 molecules/platelet) and platelet aggregation but not secretion. Platelets express 40,000-50,000 GP IIb-IIIa molecules in their surface membranes. However, resting platelets only bound approximately 5,000 D3GP3 molecules/platelet. D3GP3 binding to platelets could be increased 2-3-fold by dissociation of the GP IIb-IIIa complex with 5 mM EDTA or by occupying the fibrinogen receptor with either RGDS peptides or fibrinogen. Platelet stimulation with ADP in the absence of fibrinogen did not cause increased D3GP3 binding above control levels. These data suggest that 1) GP IIb-IIIa can exist in multiple conformations in the platelet membrane, 2) D3GP3 binding to GP IIIa can expose the fibrinogen receptor, 3) the binding of either RGDS peptides or fibrinogen causes exposure of the D3GP3 epitope, and 4) platelet activation in the absence of ligand does not induce the same conformational changes in GP IIb-IIIa as does receptor occupancy by RGDS peptides or fibrinogen.