Oxidation of polyunsaturated fatty acids by thiyl radicals derived from GSH or Cys is believed to be responsible for some of the biological damage resulting from lipid oxidation under oxidative stress. However, this has not been demonstrated in complex biological systems. In this study, we measured the formation of lipid hydroperoxides in liposomes exposed to radicals generated by γ radiation from GSH, GSSG, GSMe, Cys and Met. In the absence of proteins, the radicals oxidized the liposome lipids. In the presence of proteins, the thiyl radicals failed to react with the liposomes, even though the protein radicals efficiently oxidized the S-compounds. It appears that the thiyl and other S-radicals were effectively scavenged by the protein before initiating lipid oxidation. The results suggest that membrane lipid oxidation in vivo by thiyl radicals is unlikely to be a significant event.