THREE fibrinolytic agents have been encountered in fibrin clots. The first was described as lytic factor in an earlier communication1. It was extracted with borate from trypsin-digested clots. It has since been found that extraction can be carried out successfully with 0.9 per cent sodium chloride and other salt solutions. Active preparations contained a 17-ketogenic steroid and were inactivated by β-glucuronidase and by heating at 56° C for 30 min. The inhibitory effect of potent lytic factor on the clotting of fibrinogen by thrombin or aqueous clot extract1 was very marked. An amount of 0.0004 ml. prevented fibrin formation, while with 0.0002 ml. only a partial clot was produced. With recalcified oxalated plasma different reactions were observed. One-hundredth ml. of the same preparation induced formation of a complete clot in 30–35 sec; with 0.05–0.08 ml. partial clots formed in the same time interval, while 0-15 ml. prevented clotting entirely. The lytic activity of the factor was strongly inhibited by serum, even after the serum had been treated with 0.2 N sodium hydroxide to destroy glucuronides and then neutralized with hydrochloric acid.