A 34 year old male bitten by an adult Atheris squamiger snake developed symptoms of nausea, vomiting, diarrhea which were followed by drowsiness and impaired breathing. Local hemorrhage, edema and pain at the bite-site occurred, but no systemic bleeding or hemorrhagic diathesis developed. All clinical and laboratory parameters were in the normal range except for afibrinogenemia, thrombocytopenia and slight proteinuria. Replacement therapy (fibrinogen and platelet concentrates) and treatment of shock stabilized the patient within 2d and coagulation returned to normal. Atheris squamiger venom was subjected to biochemical and biological analysis. The LD50 of the venom was 5 mg/kg (mice, s.c.). It produced local hemorrhage corresponding to about 25% of the activity of puff adder venom (Bitis arietans). In vitro the venom had a fibrinogen-converting activity, it did not activate purified prothrombin but very likely contained a F V and Ca2+-dependent prothrombin activator. The venom exhibited strong platelet-aggregating activity, which was not inhibited by protease inhibitors and by EDTA or EGTA. The venom also aggregated acetylsalicylic acid treated platelets indicating, that the arachidonic acid pathway was not essential for activation. Rat serum rapidly inhibited the platelet-aggregating activity of the venom; human serum, however, had only a partial inhibitory effect. Preliminary experiments showed that platelet-aggregating activity may be separated from fibrinogen-converting activity by anion-exchange chromatography.