Urokinase was covalently immobilized to a cellulose hollow fiber membrane with the method of CNBr-activation. The resulted immobilized urokinase showed 19-38×10^(-3) CTA unit/cm fiber compared to 17-30×10^(-3) CTA unit/㎝ fiber of the simply adsorbed urokinase which showed the least stability during repeated use. The fibrinolytic activity of the immobilized one was very stable and the immobilized fiber promised a potential clinical use.