Anacardic acid, a salicyclic acid analog, which is a constituent of cashew nut shell, inhibited markedly the increase in cyclic AMP (cAMP) induced by prostacyclin (PGI2) in human blood platelets. It also inhibited markedly the synthesis of thromboxanB2 (TXB2) from 14C-arachidonic acid. Anacandic acid did not inhibit platelet aggregation induced by arachidonic acid but overcame the inhibition induced by PGI2. These findings indicate that anacardic acid may interact with PGI2 receptors in blood platelets and may be useful in controlling platelet functions.