Thrombin binds tightly to human platelets. The binding reaction is dependent on the thrombin concentration used. At a physiologically significant thrombin concentration, there are about 500 binding sites per platelet with an apparent dissociation constant of 0.02 u/ml. Autoradiography studies of platelets treated with labelled thrombin showed that the thrombin was located on the platelet surface. Separation of the subcellular fractions of platelets treated with labelled thrombin by density gradient centrifugation revealed that the membrane area contained over 80% of the radioactivity initially applied. Furthermore, isolated platelet membranes dind thrombin similar to intact platelets. These data suggest that the receptors for thrombin are located on the platelet membrane. Cytochalasin A, cytochalasin B or prostoglandin E1 did not have any effect on thrombin binding although these agents inhibited platelet aggregation. Thus, binding of thrombin is not sufficient for aggregation of platelets and other steps are involved. These agents do not affect the induction of stimulation but interfere at a later step in the trhombin-platelet interaction. On the other hand, hirudin completely inhibited binding of thrombin to platelets. It appears that the platelet receptor recognizes that part of the thrombin molecule on its surface which hirudin. Binding studies with serotonin loaded platelets showed a close correlation between thrombin binding and the release reaction.