Cultured mammalian cell systems are useful for examining the quantitative effects of mutagens and antimutagens on cell survival and gene mutations and the mechanisms of the interaction of two chemicals in the process of mutation induction. In the present article, the antimutagenic effects of vitamins C, E, and A, and derivatives of vitamin C on EMS-induced 6TG-resistant mutations in Chinese hamster V79 cells were examined. Vitamin C was most effective in inhibiting EMS-induced cytotoxicity and 6TG-resistant mutations. In the presence of vitamin C at a concentration of 100 micrograms/ml, EMS-induced mutations were reduced to about one-third or one-fourth of those in control cultures treated with EMS alone. Dehydro-vitamin C and iso-vitamin C also inhibited EMS-induced mutations to about one-half or one-third of the control level. The fact that vitamin C was effective in reducing EMS-induced mutations when EMS was previously incubated together with vitamin C for 3 hr suggests that vitamin C may react directly with EMS as a desmutagen and thus inactivate its mutation-inducing activity in Chinese hamster V79 cells. Vitamin E had an additive cytotoxic effect on EMS-induced cytotoxicity. This vitamin enhanced the frequencies of 6TG-resistant mutations induced by EMS. Pretreatment with vitamin E before treatment with EMS resulted in no detectable effect in modifying the EMS-induced mutations. On the contrary, vitamin A markedly enhanced EMS-induced mutation frequencies.