Cerastobin, a thrombinlike enzyme with arginine esterase activity, was purified from crude Cerastes vipera (Egyptian Sahara snake) venom. Unlike thrombinlike enzymes isolated from other snake venoms, cerastobin had a potent platelet aggregatory effect. The activation of human platelets was not related to adenosine diphosphate release and/or prostaglandin synthesis. Cerastobin showed a proteolytic activity towards protein constituents of the platelets' cytoskeleton. It hydrolyzed actin, actin-binding protein, and P235. This may explain at least a part of the aggregatory mechanism(s) of cerastobin. Electron microscopic studies of the stimulated platelets revealed changes in their morphology, including the appearance of pseudopodia, dilatation of the canalicular system with the formation of peripheral balloons, and centralization of the platelet organelles. Some inhibitors of the esteratic activity of cerastobin also inhibited its ability to aggregate platelets.