The anti-free radical action of rebamipide (2-(4-chlorobenzoylamino)-3-[2(1H)-quinolinon-4-yl] propionic acid, CAS 11911-87-6), a novel anti-ulcer agent, was investigated by electron spin resonance spectroscopy. A spin trapping method using 5,5-dimethyl-1-pyrroline-N-oxide showed that rebamipide dose-dependently decreased both the intensity of the hydroxyl radical spin adduct signal generated by the Fenton reaction and the intensity of the superoxide spin adduct generated from opsonized zymosan-stimulated neutrophils. A kinetic study showed that the second order rate constant for the reaction between rebamipide and hydroxyl radicals at pH 7.8 was 2.24 x 10(10) mol l-1 s-1. These results indicate in vitro antioxidant properties of rebamipide which is active as hydroxyl radical scavenger and inhibitor of superoxide production in neutrophils.