A γ-globulin fraction from the antiserum prepared in rabbits to a highly purified urokinase preparation was used to immunologically identify the tissue activator activity in the bladder. Using fibrin plates incorporating the fraction at a concentration of approximately 0.015per cent (W/V), the activity of urokinase standards (0.03ml of 1.5 Ploug units per ml) was completely neutralized. In a series of serial dilutions of the fraction, 100per cent specific neutralizing effect on urokinase standards (6 Ploug units per ml) was observed at a concentration of 0.5per cent of the fraction, while γ-globulin from control sera was not inhibitory even at higher concentrations. Local plasminogen activator activity of cells in the bladder tissue section, which was demonstrated by histochemical techniques, received no significant inhibitory effect with fibrin film incorporating the fraction from antiurokinase serum at a concentration of 2per cent. Plasminogen activators extracted with 0.15M KCl, 2M KCl and 2M KSCN from the bladder were assayed with the fraction from antiurokinase serum on fibrin plates. Approximately 40per cent of the activity of soluble type tissue activator (s) extracted with 0.15M KCl as vehicle was due to the activator antigenically identical to urokinase, while the others differed from that of urokinase in immunoassays. References concerning urokinase and its inhibitor are reviewed and their importance in dynamic changes of the fibrinolytic system in various pathological conditions is discussed.