Magnolol and honokiol are two position isomers isolated from the bark of Magnolia officinalis. Both inhibited the aggregation and ATP release of rabbit platelet-rich plasma induced by collagen and arachidonic acid without affecting that induced by ADP, PAF or thrombin. Aggregation of washed platelets was more markedly inhibited than that of platelet-rich plasma, while the aggregation of whole blood was least affected by both inhibitors. Thromboxane B2 formation caused by collagen, arachidonic acid or thrombin was in each case inhibited by magnolol and honokiol. The rise of intracellular calcium caused by arachidonic acid or collagen was also suppressed by both agents. Collagen-induced intracellular calcium increase in the presence of indomethacin was suppressed by magnolol. It is concluded that the antiplatelet effect of magnolol and honokiol is due to an inhibitory effect on thromboxane formation and also an inhibition of intracellular calcium mobilization.