The present investigation has failed to show any correlation between the lethal activity of Toxotoxin and the viscosity of its solutions.In vitroexperiments in the presence of mouse plasma revealed a dual character of Toxotoxin, this being partly thromboplastin and partly coprecipitant to fibrin.In vivo, the thromboplastic activity, though in itself checked by fibrinolysis, was thought to initiate the coprecipitation of Toxotoxin, thus causing the formation of a massive clot which fibrinolysis would be unable to resolve.