Summary: Fibrinolysin of a group A strain (Co) of hemolytic streptococcus does not exert its characteristic rapid action on human fibrin, if the fibrin is sufficiently pure. Such human fibrin can be rendered susceptible to the fibrinolysin by the addition of a lytic factor present in the euglobulin fraction of normal human serum. The human lytic factor also renders preformed rabbit fibrin susceptible to the group A fibrinolysin. The lytic factor contaminates the usual crude fibrin clot but is distinct from thrombin and fibrinogen.