Although calpain (the Ca2(+)-dependent protease) is widely distributed, its function is poorly understood. One cell in which it becomes activated as a consequence of activation of the cell is the blood platelet. The aim of the present study was to determine whether activation of calpain was responsible for any of the responses of platelets to stimulation. Platelets were incubated with calpeptin, a membrane-penetrating inhibitor of calpain, before being exposed to an agonist. Concentrations of calpeptin that totally inhibited the agonist- induced hydrolysis of actin-binding protein (ABP) by calpain had no effect on many other responses associated with platelet activation: phosphorylation of myosin light chain, phosphorylation of P47, platelet shape change, aggregation of platelets, secretion of granule contents, or retraction of fibrin clots. However, these concentrations of inhibitor decreased the agonist-induced generation of procoagulant activity (assayed as the ability of platelets to catalyze the conversion of prothrombin to thrombin in the presence of factor Va and factor Xa). When thrombin was the agonist, the amount of ABP that was hydrolyzed was small; only a small component of the total agonist- induced procoagulant activity was inhibited by calpeptin. When collagen was the agonist, more ABP was hydrolyzed and the amount of procoagulant activity generated was greater; calpeptin decreased the collagen- induced procoagulant activity to levels comparable with those induced by thrombin in the presence of the inhibitor. We suggest that there are at least two mechanisms by which procoagulant activity is generated on activated platelets and that the agonist-induced activation of calpain mediates one of these mechanisms. These results show that activation of calpain is a component of the stimulus-response pathway in platelets.