The rate of digestion of fibrin was measured in vitro by an isotopic technique in 140 blood samples of differing fibrinogen concentration; the assessment of fibrinolytic activity thus obtained was compared with a standard method of measurement, the dilute whole blood lysis time. The lysis time was related exponentially to the fibrinolytic activity as measured by the isotopic technique, and further was influenced markedly by alteration in the plasma fibrinogen concentration. The relevance of these observations to the use of lysis time methods for the measurement of fibrinolytic activity is discussed.