Methods are disclosed for sterilizing preparations of urokinase to reduce the level therein of one or more active biological contaminants or pathogens, such as viruses, bacteria (including inter- and intracellular bacteria, such as mycoplasmas, ureaplasmas, nanobacteria, chlamydia and rickettsias), yeasts, molds, fungi, single or multicellular parasites, and prions or similar agents responsible, alone or in combination, for TSEs. These methods involve sterilizing preparations of urokinase with irradiation.