125I-fibrinogen bound to ADP-activated fixed platelets in a saturable manner. The Scatchard plot was curvilinear but nonlinear model fitting of the data suggested that there was only one binding site with KA=4.19+/-1.3 x 10(6) M(-1) and a maximum number of binding sites of 3.9+/-1.1 x 10(4) molecules/platelet. The GPIIb/IIIa antagonists RGDS and FK633 both inhibited 125I-fibrinogen binding (50 microg/ml) in a dose-dependent manner. FK633 had a KB value of 2.5+/-0.48 x 10(7) M(-1) (Kb=39.9 nM) and an IC50, value of 64 nM, while RGDS had an KB of 2.55+/-0.76 x 10(4) M(-1) (Kb=39.2 microM) with an IC50 value of 63 microM. At concentrations below its IC50 value FK633 was a competitive antagonist of fibrinogen binding. However, at concentrations above its IC50 value it was a noncompetitive antagonist. The IC50 values of FK633 remained constant over a wide range of fibrinogen concentrations while its KB value changed-increasing from 1.8+/-0.6 x 10(7) M(-1) at 10 microg/ml 125I-fibrinogen to 4.5+/-1.6 x 10(7) M(-1) at 300 microg/ml and decreasing to 0.3+/-0.2 x 10(7) M(-1) at 2.4 mg/ml. Thus, FK633 is a reversible, noncompetitive antagonist of fibrinogen binding to the platelet GPIIb/IIIa receptor.