To estimate the amount (or potency) of the fibrinolytic “enzyme” formed or secreted by Streptococcus hemolyticus, the following titrative method has been adopted by our laboratory. Standard solutions of fibrinogen and thrombin are prepared from freshly drawn human blood by the method of Tillett and Garner.1 Serial dilutions (1:2) are made of the broth culture to be tested. To 0.5 cc. of each dilution there is added 1 cc. of the standard solution of fibrinogen. Coagulation is then brought about by the addition of 0.1 cc. of standard solution of thrombin, the fibrinous clot usually forming within 30 seconds, after which each tube is placed in a thermostatic waterbath (37°C). Readings are usually made at the end of 15 minutes, 30 minutes, 60 minutes and 2 hours. The highest serial dilution of the broth culture causing complete liquefaction of the clot by the end of 2 hours' incubation is assumed to contain one arbitrary fibrinolytic unit. From this dilution the number of lytic units per cc. of broth culture ...