A study of guinea pig and human skin in vivo has revealed that keratinocytes contain a thioenzyme which reduces radicals. This enzyme has been purified by affinity column chromatography and identified as thioredoxin reductase. In vivo and in vitro bioassays were performed by using a spin-labelled surfactant as the radical substrate, because it can diffuse through the stratum corneum and react by surface complexation with the epidermis and also on the outer plasma membrane of keratinocytes from cell cultures. Thioredoxin, the native substrate for thioredoxin reductase effectively competes for electrons with radical substrates. Nicotinamide adenine dinucleotide phosphate (NADPH) is the electron donating coenzyme in both the reduction of radicals and thioredoxin. Reduced thioredoxin has been shown to be an inhibitor of tyrosinase, whereas oxidized thioredoxin has no effect on this enzyme activity. Taken together these results indicate that the thioredoxin/thioredoxin reductase system plays an important role in preventing cell damage from UV-generated free radicals on the skin.