A platelet aggregation inhibitory protein, bitistatin, was isolated from the venom of the puff adder Bitis arietans. This protein is a single-chain peptide containing 83 amino acids and 7 disulfide bonds. Bitistatin contains the sequence arginine-glycine-aspartic acid and shows considerable homology to two previously described snake venom platelet aggregation inhibitors, trigramin and echistatin. Bitistatin inhibited human and canine platelet aggregation initiated by 10 microM ADP in vitro with IC50 values of 237 +/- 13 and 28 +/- 3 nM, respectively. In order to assess the antithrombotic potential of bitistatin, a canine model of platelet-dependent coronary thrombus formation was utilized. Injection of bitistatin at 10-100 micrograms/kg (0.7-7 nmol/kg, intravenously (i.v.] resulted in dose-dependent inhibition of both platelet aggregation ex vivo and platelet-dependent cyclical flow reductions. The effective dose to inhibit cyclical flow reductions was 30 micrograms/kg, i.v. A higher dose of bitistatin (100 micrograms/kg, i.v.) inhibited cyclical flow reductions for 160 +/- 29 min as well as attenuated ex vivo platelet aggregation. Bitistatin at 100 micrograms/kg, i.v. prolonged the bleeding time 4 x normal at 15 min post-administration but returned to normal at 3 h. Thus, in a canine model of in vivo platelet aggregation, bitistatin is an effective antiplatelet agent to inhibit periodic cyclical flow reductions. Bitistatin also exhibits reversible effects of ex vivo platelet aggregation as well as on bleeding time.