thiazinamium chloride and promethazine inhibits TxB2 synthesis by resting macrophages in a dose-dependent manner. Thiazinamium chloride inhibits TxBz synthesis but had no effect on the ingestion of zymosan particles. In contrast, chlorpromazine inhibits phagocytosis but not TxBz synthesis except at lo-3 M. Under the condition where indomethacin, a nown cyclooxygenase inhibitor, is inhibitory, promethazine but not thiazinamium chloride inhibits TxB2 synthesis from exogenous arachidonic acid. Treatment of macrophages with promethazine and chlorpromazine but not thiazinamium chloride results in a reduction in the oxidative burst during phagocytosis. Furthermore, the ability of thiazinamium chloride to selectively inhibit arachidonic acid metabolism may contribute to its bronchodilator/antiallergic activity.