SP-22 was found to be a substrate protein of a mitochondrial ATP-dependent protease in bovine adrenal cortex. Its amino acid sequence was homologous to that of some prokaryotic and eukaryotic proteins such as thioredoxin peroxidase (formerly called thiol-specific antioxidant) in yeast and mammalian brains and the C22 component of alkyl hydroperoxide reductase in Salmonella typhimurium. In the present study, we found SP-22 to have the ability to scavenge reactive oxygen species, thus protecting radical-sensitive proteins such as tryptophan hydroxylase, glutamine synthetase and hemoglobin from oxidation. The protecting activity was enhanced by the addition of horse serum. The "serum factor(s)" seemed to be protein(s), since the physiological roles of SP-22 in adrenocortical mitochondria are discussed.