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