Adult male and female acatalasemic (C3H/AnLCsbCsb),hypocatalasemic (C3H/AnLCscCsc) and normal mice of C3H strain fed on regular laboratory chow for 15 months showed an increased incidence of spontaneous mammary tumor in the decreasing order of female acatalasemic, male acatalasemic, female hypocatalasemic and male hypocatalasemic mice. Normal mice did not develop mammary tumor. We conducted a prospective study with female acatalasemic mice, which showed the highest incidence of mammary tumor, to examine the preventive effect of vitamin E on mammary tumor. Female acatalasemic mice were fed on vitamin E-deficient (28 animals) and vitamin E-supplemented diet (25 animals) for 29 months. The incidence of mammary tumor in mice given the vitamin E-supplemented diet was 47%, while that in mice given vitamin E-deficient diet was 82% (P < 0.002). Mammary tumors were apparent after 9 months of vitamin E deprivation and after 14 months of vitamin E supplementation. Female normal mice did not develop mammary tumor during a comparable period of time. The mean catalase activity of mammary gland in acatalasemic mice was 18.8% of that in normal mice. The results indicate that vitamin E protects acatalasemic mice against the development of mammary tumor.