Five phenolic compounds and pyridoxine were studied for their activities as both scavengers of superoxide anions and inhibitors of lipid peroxidation. The superoxide anions were generated in a phenazin methosulfate-NADH system and were assayed by the reduction of nitroblue tetrazolium. The superoxide anion scavenging activities of verbascoside and alizarin yellow R were the strongest, followed by those of caffeic acid and phloridzin; vanillin and pyridoxine exhibited the weakest activity. The concentration values yielding 50% inhibition of lipid peroxidation in mouse liver microsomes were 10(-5) M for verbascoside, 10(-4) M for alizarin yellow R and caffeic acid, and 10(-3) M for phloridzin; vanillin and pyridoxine had almost no antioxidative activity. The inhibition of lipid peroxidation by these individual compounds was much weaker than by butylated hydroxyanisole. The results showed that phenolic compounds and pyridoxine have more than one mechanism of action for free radicals and are able to suppress free radical processes at two stages: the formation of superoxide anions and the production of lipid peroxides.