We investigated the effects of the traditional Korean prescription, Danggijakyaksan (DJS) on antiplatelet activity in human platelet suspensions. The effect of oriental medicinal prescriptions, Danggijakyaksan consisting of 6 herbes of Paeoniae Radix (2 g), Poria Cocos (1.33 g), Angelicae Sinensis Radix (1 g), Cnidii Rhizoma (1 g), Atractylodis Macrocephalae Rhizoma (1.33 g) and Alismatis Rhizoma (1.66 g), was studied. In this study, the mechanism involved in the antiplatelet activity of DJS in human platelet suspensions was investigated. Danggijakyaksan did not significantly affect the thromboxane synthetase activity of aspirin-treated platelet microsomes and DJS (20 and 40 microg/mL) significantly inhibited [3H]arachidonic acid (AA) released in collagen-activated platelets but not in unactivated-platelets. Nitric oxide (NO) production in human platelets was measured by a chemiluminesence detection method in this study. Danggijakyaksan did not significantly affect nitrate production in collagen (10 microg/mL)-induced human platelet aggregation. On the other hand, various concentrations of DJS (10, 20, and 40 microg/mL) dose-dependently inhibited [3H]inositol monophosphate (IP) formation stimulated by collagen (10 microg/mL) in [3H]myoinositolloaded platelets at different incubation times (1, 2, 3, and 5 min). It is concluded that the antiplatelet activity of DJS may possibly be due to the inhibition of phospholipase C activity, leading to reduced phosphoinositide breakdown, followed by the inhibition of thromboxane A2 formation, and then inhibition of [Ca2+]i mobilization of platelet aggregation stimulated by agonists.