Streptokinase was infused for 103 +/- 25 hrs. in 27 patients (6 with deep vein thrombosis of the leg, 7 with thrombosis of Vv. axillaris and subclavia, 9 with acute massive pulmonary embolism, 3 with chronic artery occlusion, 2 with thrombosis of the retinal vein). Blood coagulation studies were performed repeatedly. Anticoagulation (prolongation of thrombin time to 1 1/2-2 1/2 of normal) could be achieved without additional heparin by reduction of streptokinase to doses as low as 20 000 U/h. Reptilase time correlated significantly with thrombin times. Following 24 hrs. of infusion, PTT was less than 50 sec. and Quick test less than 50% of normal in most instances. No correlation was found between PTT or Quick test and factor II, V, VI, X activities during streptokinase infusion. Bleeding was observed in 18 patients. The infusion was stopped because of bleeding in 3 cases. None of the coagulation tests performed in this study correlated with the incidence of bleeding. Hemoglobin concentration decreased 2,6 +/- 1,6 gr% and this decrease could no be explained by blood lesses.