A new method for determination of the rate of plasma fibrinolytic activity is presented. The method is based on the euglobulin clot-lysis test, modified by the addition in vitro of a small amount of normal I-labeled fibrinogen to the patient's eublobulin fraction. The rate of fibrinolysis is determined by the release of radioactivity from the clotted euglobulin fraction as a function of time. The rate is expressed as the fibrinolysis constant, which is the mathematical value for the fibrinolysis curve. Mean normal value for the fibrinolysis constant for 17 normal adult volunteers was 0.14 +/- 0.09 (2 SD). The major advantages of this technic are that it is not necessary to rely on complete dissolution of the clot for the end point, the majority of the euglobulin clot is the individual's own fibrin(ogen), and a quantitative and kinetic index of overall fibrinolytic activity can be obtained.