Glutathione (GSH) dependent protection against oxidative damage of human red cell membrane was examined. An artificial system was used in which chloroform/methanol-extracted red cell lipids, in the form of liposomes, were subjected to attack by a peroxidation system consisting of ascorbate-Fe3+. Human erythrocytes contained a nondialyzable factor, completely inactivated by heating in a boiling water bath for 3 min, which showed GSH-dependent inhibition against lipid peroxidation and was devoid of GSH peroxidase activity. On the other hand, GSH-S transferase, highly purified by affinity chromatography, had no inhibitory activity. These findings strongly indicate that the GSH- dependent protection against lipid peroxidation of human red cell membrane is mediated by one or more proteins other than GSH peroxidase and GSH-S transferase.