In nine patients suffering from organophosphate intoxication, platelet function and blood coagulation parameters were investigated. Thrombocyte function was impaired in all patients, characterized by a diminished platelet shape change. Platelet shape change was also inhibited in rats and after oral administration of 10 mg/kg parathion. Thrombocytopenia and coagulation abnormalities (diminished fibrinogen, plasminogen and anti-thrombin III) were more pronounced in cases with severe intoxication. In five of nine patients a marked bleeding tendency was observed. The bleeding tendency in organophosphate intoxication is probably mainly caused by the defective platelet function. Patients with this intoxication should receive heparin only for special indications; haemoperfusion is of doubtful value since this procedure may itself produce haemostatic defects.