The biological impact of selenium on the levels of antioxidant enzymes in Wistar rats bearing mammary tumor induced by dimethylbenz(α)anthracene was investigated. Control rats and tumor-bearing rats were fed a normal diet or one containing 5 mg sodium selenite/kg diet from the day of tumor induction. The reduced levels of ceruloplasmin, ascorbic acid, and α-tocopherol seen in the serum of tumor-bearing rats on the normal diet were found to be increased by the selenium treatment. The activities of superoxide dismutase and catalase in tumor-bearing rats were decreased significantly when compared with those of control rats, whereas selenium administration caused a considerable recovery of the activities of these enzymes in the rats with tumors. The increase in the levels of these enzymes was found to be predominantly significant in the liver. These observations clearly suggest an antioxidant role for selenium in experimental mammary tumor. © 1995, SOCIETY FOR FREE RADICAL RESEARCH JAPAN. All rights reserved.