Anti-platelet and vasodilating actions of OP-41483, a derivative of prostacyclin, were studied experimentally and clinically. The ADP-induced human platelet aggregation was significantly inhibited in vitro, the rate being 59% with a dose of 3 micrograms/ml, 75% with 6 micrograms/ml and over 90% with 18 micrograms/ml or more. A significant reduction in deposition of platelet and mural thrombi on the chemically injured luminal surface of the canine femoral vein was observed by treatment with topical administration of the solution (10 micrograms/ml) and/or intravenous infusion (10 ng/kg/min). The blood flow rate of the normal canine femoral artery and the anterior or posterior tibial artery of patients with peripheral arterial occlusive disease at the ankle was moderately increased in cases of intravenous infusion of the compound at a rate of 10 ng/kg/min.