Pure catalase from bovine liver was used to study the photosensitive effect by two photosensitizors hematoporphyrim and riboflavin. The change of enzyme protein conformation in solution was determinted by ultraviolet absorption difference spectra. The activity of catalase was measured by oxygen electrode.Catalase activity was inhibited in the presence of light and photosensitizor. The inhibition increased with the increasing concentration of photosensitizor and illumination time. Absorption spectra were shifted and the shape of absorption peak was modified in the cata- lase-photosensitizor reaction. A nagative peak of 229 nm and a 236—240 nm peak in UV difference spectra were observed by the action of hematoporphyrin and riboflavin, respectively. The results indicate that the activity inhibition of catalase induced by photosensitive oxidation was related to the change of enzyme protein conformation. It is suggested that the photosensitive inactivation of catalase may be due to the photodynamic injury caused by the active oxygen produced from photosensitive reaction.