An intermittent dosage scheme of streptokinase (standard initial dose 600,000 units SK infused over 30 min and repeated injections of 250,000 units SK at 24 hr intervals) was applied during 4 days in 9 patients with chronic obliterative arterial disease and in 8 patients with venous occlusion. Each dose of streptokinase produced an immediate fall in plasminogen to 17% (SEM 5.1) of the initial value, the level then rose to 50% (SEM 5.4) within 24 hr. Lowered levels of antiplasmin and fibrinogen (both less than 40% of the initial values) were maintained. This safe level of fibrinogen was maintained despite brief but high peaks of plasmin activity after each injection of SK. A parallel increase of the thrombin time and the fibrin (ogen) degradation products was obtained following each infusion. No bleeding was observed. The relative therapeutic effect of intermittent infusions of streptokinase has still to be compared with the continuous administration of streptokinase in controlled clinical trials.