Seven ginsenosides, namely Rg6 (1), F4 (2), Rk3 (3), Rh4 (4), Rs3 (5), Rs4 (6) and Rs5 (7) isolated from processed ginseng were evaluated for their effects on platelet aggregation induced by adenosine diphosphate (ADP), collagen, arachidonic acid (AA) and U46619 (thromboxane A2 mimetic drug). Ginsenosides Rg6, F4 and Rk3 showed inhibitory activity (IC50 = 76 microM, 114 microM and 128 microM, respectively) on AA-induced platelet aggregation. The corresponding IC50 values were comparable to that of acetylsalicylic acid (ASA) (63 microM). Compared to ASA (IC50 = 468 microM) ginsenosides Rg6, F4, Rk3 and Rh4 were found to be more inhibitive (IC50 = 286 microM, 87 microM, 187 microM and 119 microM, respectively) against U46619-induced aggregation. On the other hand, most of the ginsenosides (Rg6, F4, Rh4, Rs3, Rs5) showed negligible effects on ADP and collagen-induced platelet aggregation. The acetylated ginsenosides (Rs3, Rs4 and Rs5) had only mild effects on aggregation induced by four stimulators.