The new water-soluble ammonium-analog of alpha-tocopherol (vitamin E) (compound 1: 3,4-dihydro-6-hydroxy-N,N, N-2,5,7,8-heptamethyl-2H-1-benzopyran-2-ethanaminium 4-methylbenzenesulfonate) and its tertiary amine derivative (compound 2: 3,4-dihydro-2-(2-dimethylaminoethyl)-2,5,7,8-tetramethyl-2H-1-benzopyran -6-ol hydrochloride) were investigated as scavengers of oxygen-derived free radicals. Compounds 1 and 2 were at least 40 times more potent inhibitors of Fe-driven heart microsomal lipid peroxidation than Trolox. While the alpha-tocopherol analogs had the same potency as scavengers of xanthine/xanthine oxidase-generated superoxyl radicals, the thiol compounds D,L-penicillamine and N-2-mercaptopropionyl glycine reacted at a much slower rate. The O-acetyl derivatives of compounds 1 and 2 were not scavengers of superoxyl radicals. Considerable differences between the alpha-tocopherol analogs were observed in their competition with 2-deoxyribose for hydroxyl radicals (OH.). Compound 2 was equipotent with Trolox and thiourea, whereas the reactivity of these substances was diminished by more than 30% as compared to compound 1. Although showing lower reactivity, the O-acetyl derivatives of compounds 1 and 2 were active nevertheless as OH.-scavengers. The previously reported high potency of compound 1 in reducing infarct size during myocardial ischemia/reperfusion appears to be due to its radical-scavenging properties, likely to be enhanced by its previously described cardioselectivity.