The possibility that thrombin acts on platelets by a mechanism other than proteolysis was investigated. The proteolytic site of thrombin was modified with phenylmethylsulfonyl fluoride (PMSF). This modified enzyme did not induce platelet aggregation or the platelet release reaction. Platelets were then incubated with the inactivated enzyme (PMS-thrombin) and later with active thrombin. In this sequence of incubation, PMS-thrombin enhanced not only platelet aggregation induced by active thrombin but also the thrombin-induced release reaction. Preincubation with PMS-thrombin was essential for this enhancement as the inhibited enzyme did not affect aggregation if added after active thrombin. The effect of PMS-thrombin was limited to thrombin-induced reactions of the platelet. The inhibited enzyme had no effect on aggregation induced by adenosine diphosphate or collagen, or on thrombin-induced coagulation of fibrinogen. These results suggest (1) that both proteolytic and binding sites for thrombin are present on the human platelet plasma membrane; and (2) that interaction of thrombin with the binding site potentiates the activity of the proteolytic site.