The anti-platelet activity of ilexoside D isolated from the roots ofIlex pubescens Hook. et Arn. was investigated inin vitro andex vivo models of platelet aggregation induced by ADP, thrombin or collagen in rats.In vitro ilexoside D inhibited more effectively platelet aggregation induced by ADP and thrombin than by collagen as compared with aspirin.Ex vivo ilexoside D also inhibited platelet aggregation induced by ADP and collagen, but not by thrombin, and the inhibitory action of ilexoside D was more effective than that of aspirin. However,in vitro ilexoside D inhibited very poorly the generation of malonyldialdehyde, which is known to be concomitantly released with thromboxane A2 during platelet aggregation. These results suggest that the anti-platelet activity of ilexoside D may not be responsible for prostaglandin synthesis in platelets.