In thrombin-stimulated platelets (XiibP3 integrin engagement triggers both phosphatidylinositol 3',4'-6wphosphate synthesis and calpain activation. We checked the possible involvement of calpains in phosphatidylinositol 3-kinase signalling pathway using a cell permeant specific inhibitor of calpains, calpeptin. In conditions where thrombin-induced platelet aggregation and secretion were not impaired, we found a dosedependent inhibition of phosphatidylinositol 3,4-6wphosphate synthesis by calpeptin from 50 jig/ml. Moreover, pretreatment of platelets by both calpeptin and the peptide RGDS, an inhibitor of fibrinogen binding to activated an b p 3 integrin, did not induce additive effects on phosphatidylinositol 3,4-/>/vphosphate inhibition. Finally, the p85 regulatory subunit of phosphatidylinositol 3-kinase was still translocated to the cytoskeleton in calpeptintreated platelets. These data indicate that calpains are involved in the regulation of a 1Ib p 3 integrin-dependent phosphatidylinositol 3-kinase signalling pathway.