Crude venom from Eristocophis macmahoni was demonstrated to exert a potent inhibition of human blood platelet aggregation mediated by adenosine diphosphate (ADP), platelet activating factor (PAF) and arachidonic acid (AA). The venom caused lysis of the platelets, however, the red blood cells were not lysed by the venom. Substantial oedema was produced upon injection of the venom into the rat hind paw. Contrarily, the intraperitoneal injection of the venom to the rats caused an inhibition of the carrageenin-induced rat paw oedema. However, an 100% lethality within 24 h was observed with a dose of 40 mg/kg body weight. The venom was fractionated by reverse phase high pressure liquid chromatography (HPLC) and the fractions were analyzed for their effect on ADP-induced platelet aggregation. The fraction eluted at 15.5 min (20% acetonitrile concentration) exhibited an inhibitory effect of several-fold greater potency than that of the crude venom. Fractions eluted at 18.5 min (25.4% acetonitrile concentration) and onward showed a proaggregatory but insignificant effect. It is suggested that although the venom contains pro aggregatory components, inhibition of platelet aggregation seems to be its predominant activity.