1. A synthetic peptide disulfide, Gln-Val-Val-Cys(NpyS)-Gly-NH2 (P1) inhibited thrombin and plasmin-induced platelet aggregation and cleavage of aggregin. P1 did not inhibit platelet aggregation induced by other agonists nor did it inhibit shape change. 2. P1 also inhibited purified platelet calpain II. 3. The correspondence between the molecular structure of P1 and inhibitory sequence of the peptide in domain 2 of high molecular weight kininogen has shed light on the molecular nature of the cellular mechanism underlying thrombin- and plasmin-induced platelet aggregation and the inhibition by P1. 4. P1 may prove to be useful in designing and improving future protocols of thrombolytic therapy to prevent reocclusion. P1 may also have a role in inhibiting thrombin formed during angioplasty and thus preventing restenosis.