A new antithrombotic drug, cilostazol (6-[4-(1-cyclohexyl-1 H-tetrazol-5-yl)butoxy]-3,4-dihydro-2(1H)-quinolinone, OPC-13013) was studied for its inhibitory effect on platelet aggregation in vitro in various experimental animals and man and in dogs ex vivo, for its effect to disperse platelet aggregates in vitro in rabbits and man and for its antithrombotic effect in vivo using its effect to prevent death due to the formation of pulmonary thrombi in mice. Cilostazol produced a potent inhibition of platelet aggregation both in vitro and ex vivo and a dispersion of platelet aggregates in vitro. The mode of action of cilostazol was different from that of acetylsalicylic acid (ASA) in that cilostazol inhibits not only secondary platelet aggregation but also primary platelet aggregation induced by aggregating agents such as adenosine diphosphate (ADP). The drug potently prevented death due to pulmonary thrombosis by platelet aggregates in mice in vivo. Unlike ASA which prevented only death due to collagen-induced platelet aggregation, cilostazol prevented both collagen- and ADP-induced platelet aggregation. These results suggest that cilostazol is a promising antithrombotic drug.