Pyruvate efficiently protected V79 Chinese hamster cells against the lethal effects of hydrogen peroxide. Protection was also provided by other alpha-ketoacids, such as alpha-ketobutyrate, alpha-ketoglutarate and alpha-ketoadipate, although higher concentrations were required. The corresponding beta-ketoacids had no effect. The results indicate that pyruvate and other alpha-ketoacids possess antioxidant activity in vitro and, probably, in vivo.