The hydroxyl radical (.OH) in the presence of molecular oxygen (O2) fragmented BSA and formed distinct peptides as did hydrogen peroxide catalysed by copper. .OH in the absence of O2 caused protein crosslinking. The selective fragmentation was ascribed to cleavage at proline residues, a mechanism consistent with an increase in the proline/glutamate ratio. Other radicals, such as peroxyl and superoxide radicals caused no change in molecular weight of the protein. Peroxyl radicals and .OH were, however, able to cause some change in conformation of the protein which increased its susceptibility to hydrolysis and oxidation might thus be implicated in protein turnover. Mitochondrial monoamine oxidase (MAO) was also susceptible to free radical cleavage but lipids were able to compete with the protein for radical species. Peroxides also seemed to be involved in the cleavage of MAO and this was evidenced by a "post irradiation effect" and the ability of the hydroperoxy radical to cause degradation; in contrast to the case with BSA. In whole cells, depletion of the antioxidant GSH and inhibition of catalase increased endogenous protein degradation. Lens protein autolysis was increased under conditions where thiol autoxidation was permitted but glycosylation of protein caused a decrease in breakdown by extrinsic hydrolases. These results indicate that free radicals, possibly also associated with lipids may play a role in the regulation of protein turnover but that secondary modifications may promote the accumulation of damaged protein.