Tocopherols are present in significant amounts in vegetable oils used in human foods. The most prevalent tocopherols in foods are the alpha, beta, gamma, and delta variants with (RRR) stereochemistry. Tocopherols are lipophilic phenolic antioxidants, produced by plants. In the United States, gamma-tocopherol is the most prominent dietary tocopherol due to its high amount in the dominant commercially produced vegetable oils such as soybean, corn, and cottonseed. In this report, experiments were designed to study the inhibitory effect of mixed tocopherols against N-methyl-N-nitrosourea-induced mammary tumor growth in female Sprague-Dawley rats. Beginning at 21 days of age, rats were treated with a single intraperitoneal injection of 50 mg/kg body weight of N-methyl-N-nitrosourea. One wk later, the rats were fed experimental diets containing 0 or 0.1% mixed tocopherols containing over 50% gamma-tocopherol. At 9 wk after N-methyl-N-nitrosourea treatment, all rats were evaluated for inhibition of mammary tumor growth and proliferating cell nuclear antigen. Dietary administration of mixed tocopherols significantly suppressed mammary tumor growth (P < 0.05) and proliferating cell nuclear antigen (P < 0.01) and also moderately suppressed tumor multiplicity. The treatment increased the serum levels of gamma- and delta-tocopherols without affecting the body weight. The results of this study suggest that mixed tocopherols may be safe and effective agents for the prevention of breast cancer.