The fibrinolytic activity of blood was measured in control subjects and patients with idiopathic deep venous thrombosis by a new method, which was based on the rate of release of radioactivity in vitro from clotted blood to which 125I-fibrinogen was previously added. Blood was obtained without application of a venous tourniquet. The results are expressed as the amount of fibrin lysed per unit of time and are not affected by the fibrinogen concentration, a drawback inherent in previous techniques. The mean value (+/- standard deviation) of fibrinolytic activity was 0.464 +/- 0.158 mg of fibrin lysed per hour in the control subjects and 0.194 +/- 0.089 mg of fibrin lysed per hour in the patients. This difference is significant (p less than 0.01) and indicates a considerable reduction in fibrinolytic activity in patients with idiopathic deep venous thrombosis. This has been demonstrated without the need to stimulate the release of plasminogen activator by applying a venous tourniquet as done in previous studies.