Aspartate aminotransferase from a water soluble fraction of pig heart muscle was irradiated with ⁶⁰Co γ-ray, 274 and 380 nm light in aqueous solutions with or without radioprotective amines, i.e. 2-mercaptoethylamine (MEA) and 2-aminoethylisothiuronium bromide (AET). For γ-irradiation, the enzyme was quite sensitive (D₃₇, 37.5 krad) and was effectively protected by the amines against the radiation-inactivation. The change in protein structure judged from gel electrophoretic pattern was also improved by MEA. The 380-nm irradiation caused inactivation, which was least protected by MEA, without serious changes in the protein structure. Photochemical degradation of pyridoxal phosphate was supposed to be responsible for the inactivation. MEA protects apoenzyme probably as a radical scavenger.