Protein hydroperoxides constitute a potential hazard to living organisms because of their direct reactivity with a variety of biomolecules and the ability to decompose to free radicals. This study addressed the possibility of enzymatic removal of hydroperoxide groups from proteins, peptides and amino acids peroxidized by gamma radiation. At neutral pH and 37°C, selenium glutathione peroxidase accelerated reduction of peroxidized insulin and valine, but was ineffective with the larger BSA and lysozyme molecules. The enzyme also increased the rate of glutathione-induced reduction of peroxidized BSA after treatment with proteinase K, suggesting that size of the peroxidized molecule plays a role in the catalysis. Phospholipid glutathione peroxidase, lactoperoxidase and ebselen did not accelerate the decomposition of protein or amino acid hydroperoxides. Cysteine and methionine were the only 2 of 20 amino acids tested able to increase the rates of spontaneous decay of the protein hydroperoxides. It appears that much of the slow decay of protein hydroperoxides generated in cells exposed to hydroxyl or peroxyl radicals may be due to intramolecular reactions, with little assistance from peroxidases.