Male rats of the Sprague-Dawley strain were fed semisynthetic diets containing either 20% beef fat or corn oil with and without 20% wheat bran. Half of the animals received four weekly doses and the other half received eight weekly doses of dimethylhydrazine, 30 mg/kg, by intragastric intubation. The percentage of rats with tumors of the colon of all types was significantly higher in animals fed no bran than in those fed bran. Likewise, the percentage of rats with polypoid neoplasms of the colon was higher in rats fed no bran, but there was no significant difference in the percentage of rats with malignant tumors of the colon with respect to the feeding of bran. No significant differences were found between rats fed corn oil and those fed beef fat with respect to either the incidence or the kinds of colon tumors. Malignant tumors of the colon, causing death, occurred earlier in rats fed corn oil as compared to those fed beef fat. The percentage of rats with tumors of the colon and the numbers of tumors per tumor-bearing rat were significantly increased in rats given eight doses of dimethylhydrazine or compared to those given four.