Platelet activation is originated by the intracellular or/and extracellular Ca^(2+). Agonist-induced Ca^(2+) entry through a plasma-membrane pathway has been reported repeatedly, but the mechanisms has proven harder to elucidate. Recently, a number of natural products have been isolated from medicinal plants and marine organisms and have proved to be useful chemical tools for resolving the mechanism of cellular functions. In an attempt to understand the mechanism of platelet activation in Bupleuri Radix, we have studied some aspects of the isolation of active components and their dependence of external Ca^(2+) on platelet activation. Acetone extract of Bupleuri Radix has the most activity on platelet activation and it's active components were identified as saikosaponin a and d. Their optimal concentration was respectively 20 ug/ml and 5 ug/ml and their platelet activation was not dependent on external Ca^(2+), whereas optimal concentration of each agonist was arachidonic acid (10 uM), collagen (10 ug/ml), thrombin (0.1 unit/ml), PAF (5 uM), PMA (5 uM), ionophore A23187 (2 uM) and their dependence of external Ca^(2+) on platelet activation appeared to thrombin$gt;collagen≥PAF$gt;PMA$gt;arachdonic acid$gt;ionophore A23187. These results suggest that saikosaponin is different from each agonists in the dependence of external Ca^(2+) on platelet activation.