Purpurogallin, a flavinol obtainable from oak nutgalls, prevented lysis of human erythrocytes exposed to an azo-initiator of peroxyl radicals [2,2'-azo-bis (2-amidinopropane) dihydrochloride] at 37 degrees C. The percentage of this inhibition of erythrocyte lysis varied with the concentration of purpurogallin. In this system, the effect of purpurogallin surpassed those of lactosylphenyl-trolox, trolox and ascorbate, while mannitol, superoxide dismutase and/or catalase were without effect. While other concurrent mechanisms may be involved in purpurogallin action, it would appear that the amphipathic (both hydrophilic and lipophilic) nature of this flavinol and of trolox and lactosylphenyl-trolox, is an important factor. In fact, these three antioxidants, with purpurogallin as the best red cell protector here, excelled the protective effects of the other more hydrophilic antioxidants.