Fibrinolysin (Thrombolysin) has been used in various dosage schedules for thromboembolic disorders, but its clinical value has not been established. In this study, a single large intravenous dose was given at onset of anticoagulant therapy to 170 patients with thrombophlebitis, pulmonary embolism, or acute myocardial infarction. Only in the group treated for pulmonary embolism was the mortality significantly reduced from that observed in patients receiving only anticoagulants. Toxicity attributable to fibrinolysin was minor except for four patients who developed serum hepatitis. It is concluded that an initial infusion of fibrinolysin during anticoagulant therapy is unnecessary for thrombophlebitis and useless for acute myocardial infarction, but advisable for pulmonary embolism if the hazard of serum hepatitis is eliminated.