A single dose of 10 mg of 7,12-dimethylbenz[a]anthracene (DMBA), administered to rats through intragastric intubation, was sufficient to induce many biochemical and histopathological changes in their mammary tissue. Significant increases were observed in the activity levels of the enzymes acid ribonuclease, 5-nucleotidase, alkaline phosphatase, and beta-glucuronidase in mammary tissue homogenates of DMBA-treated rats after an experimental period of five months. Histopathological studies of the mammary tissue also revealed malignant epithelial tumors (cribriform carcinoma) induced among 85% of the treated rats, with an incidence of 4 tumors in 12 mammary glands. Nevertheless, administration of 30% soybean in the diet of rats or 5,000 ppm ascorbic acid in their drinking water in addition to DMBA revealed a significant chemoprotective effect against the carcinogenesis induced by DMBA alone. This chemoprotective effect was demonstrated by the normalization of the activity levels of the enzymes studied in mammary tissue homogenates, because most of the enzymes were maintained at near the levels in the control animals. The incidence and number of tumors were also decreased. Cribriform carcinoma was observed in 50% of the rats, and the incidence of the affected glands was 2 in 12 mammary glands among both groups. On the other hand, a less chemoprotective effect was observed due to Vicia faba administration.