MAN, other primates and guinea pig are the only mammals that are known to be unable to synthesize L-ascorbic acid; thus they require vitamin C in their diet to prevent scurvy. The rat, a typical species that is independent of a dietary source of the vitamin, synthesizes L-ascorbic acid from D-glucose as follows1–6: D-glucose → D-glucuronolactone → L-gulonolactone → L-ascorbic acid. Recent studies4–6 show that guinea pigs are unable to convert L-gulonolactone to L-ascorbic acid, a step which is catalysed in rats by enzymes present in liver. This communication reports that man and monkey also cannot convert L-gulonolactone to L-ascorbic acid.