Clove oil inhibited human platelet aggregation induced by arachidonic acid (AA), platelet-activating factor (PAF) or collagen. Clove oil was a more effective inhibitor for aggregation induced by AA and PAF (IC50:4 and 6 microM respectively) than collagen (IC50: 132 microM). The in vivo experiments in rabbits showed that clove oil (50-100 mg/kg) afforded 100% protection against PAF (11 mg/kg, i.v.) and 70% protection against AA (2.0 mg/kg, i.v.)-induced thrombosis and shock due to pulmonary platelet thrombosis. It also inhibited thromboxane-A2 and 12-HETE production by human platelets incubated with [C14] AA. These results are indicative that clove oil is inhibitory of platelet aggregation and thromboxane synthesis and may act as anti-thrombotic agent.