An antibody specific to the calpain cleavage site in talin, a cytoskeletal protein, was produced. This antibody selectively recognizes the C-terminal 200-kDa fragment generated when talin is digested by calpain and does not react at all with intact talin or the N-terminal 47-kDa fragment. To assess the involvement of calpain in the integrin-mediated signaling pathway, the effect of limited proteolysis of talin by calpain on platelet activation and aggregation was analyzed using this antibody. It was revealed that thrombin-stimulated platelet aggregation accompanies the autolytic activation of mu-calpain and the accumulation of the mu-calpain-generated 200-kDa fragment of talin. These changes were blocked by RGDS peptide which inhibits the binding of fibrinogen, an adhesive ligand, to the major integrin in platelets, alpha IIb beta 3, while RGES peptide, which has no fibrinogen-binding-inhibitory activity, had no effect. Membrane-permeable calpain inhibitors calpeptin and E-64d inhibited platelet aggregation, mu-calpain activation, and the limited proteolysis of talin. These results strongly suggest that calpain is involved in the integrin-mediated signal transduction pathway.