The effect of a 30-d pretreatment with vitamin C (L-ascorbic acid) in the drinking water and vitamin E (all-rac-alpha-tocopherol) in the diet on the clastogenic activity induced by X-rays and 6-mercaptopurine was investigated in female ICR/Jcl mice by the bone-marrow micronucleus test. Prefeeding with vitamin E-deficient diets led to a significant decrease in serum vitamin E concentration and to an enhancement of micronucleus formation by X-rays in bone marrow cells. Although dietary supplementation with vitamin E significantly increased the vitamin E concentration in serum, it did not affect the frequency of X-ray-induced micronuclei. Treatment with a high level of vitamin C in drinking water was effective in protecting against micronucleus formation by X-rays. The increase in micronucleus frequency in the vitamin E-deficient mice compared with the mice fed vitamin E-normal diets was no longer observed when a high level of vitamin C in drinking water was given simultaneously. The most efficient protective action against X-rays was observed when vitamin E-supplemented diets and a high level of vitamin C in drinking water were used together as a pretreatment. Any combination of the vitamins did not affect the micronucleus induction by 6-mercaptopurine.