We investigated the in vitro effect of modified Je-Ho-Tang (MJHT) hot-water extract on platelet aggregation and activation induced by agonists in human whole blood, and on platelet adhesion to a collagen-coated surface under flow shear stress conditions. MJHT extract potently inhibited the collagen-induced platelet aggregation in human whole blood in a dose-dependent manner, with a half-maximal inhibitory dose (IC(50)) value of 526 microg/mL. We accessed several markers of platelet activation using receptor expression on platelet membranes, including GPIIb/IIIa-like expression (PAC-1) and P-selectin (CD62); we monitored the intracellular calcium mobilization response by flow cytometry in healthy subjects. A significant decrease in PAC-1 (P=0.002), CD62 (P=0.002), and intracellular calcium mobilization (P<0.001) were seen in the presence of MJHT extract. In addition, we have used image analysis to study human platelet adhesion to collagen under physiologic flow conditions in a perfusion chamber. MJHT extract markedly decreased platelet adhesion to the collagen-coated surface. These results show that MJHT has potent anti-platelet activity.