Viral inactivation in superoxide dismutase (SOD) derived from human red cells was carried out by ultraviolet light C (UVC) irradiation. With 400J/m² UVC irradiation, the titer of canine parvovirus (CPV, a nonenveloped virus), M13 bacteriophage (M13, a nonenveloped phage) and vesicular stomatitis virus (VSV, an enveloped virus), which were spiked into SOD solution, were reduced by >4.6log₁₀ (detection limit), 7.0log₁₀ and 6.2log₁₀, respectively. The SOD activity was maintained and the band pattern of SOD on an electrophoresis gel was not changed even by 1000J/m² UVC irradiation. These results indicate that UVC irradiation is a promising method for the inactivation of both enveloped and nonenveloped viruses in SOD preparations while maintaining its activity.