When the rate of fibrinolysis in vitro was measured by an isotopic technique, it was found that the amount of fibrin digested after incubation for three hours correlated closely with the overall rate of the digestion process. It is suggested that, using an isotopic technique, estimation of fibrin digestion after three hours' incubation provides a useful method for measuring the fibrinolytic activity of blood. A recommended technique is described and its advantages are discussed briefly.