Cobra venom phospholipase A2 induced a biphasic effect on washed rabbit platelets. The first phase was a reversible aggregation which was dependent on stirring and extracellular calcium. The aggregation and thromboxane B2 formation were inhibited by indomethacin, mepacrine, tetracaine and imipramine, while PGE1 and sodium nitroprusside inhibited only the aggregation, but not the thromboxane B2 formation. The second phase was an inhibitory effect on platelet aggregation induced by arachidonic acid, PAF, ADP or collagen but not that by thrombin or ionophore A23187. The longer the incubation time of cobra venom phospholipase A2 with platelets, the more the inhibitory effect. The aggregating and anti-aggregating effects could be overcome by bovine serum albumin. Lysophosphatidylcholine (Lyso-PC) and arachidonic acid showed synergistic inhibition in platelet aggregation. Lyso-PC decreased thromboxane B2 formation in platelets formed by collagen. The inhibitory effect of Lyso-PC on platelet aggregation was more marked at lower calcium concentrations. It is concluded that the aggregating effect of exogenous addition of venom phospholipase A2 is due to thromboxane formation and the antiplatelet effect is similar to those produced by arachidonic acid and lysophosphatidylcholine.