The effect of dietary wheat bran and dehydrated citrus fiber on carcinogenesis of the colon and small intestine was studied in male F344 rats. Weanling rats were fed semi-purified diets containing 5% fat and 15% wheat bran or citrus fiber. At 7 weeks of age, all animals, except vehicle-treated controls, received weekly sc injections of 8 mg azoxymethane (AOM)/kg body weight for 10 weeks. The AOM- or vehicle-treated groups were autopsied 20 weeks after the last injection of AOM. The animals fed the wheat bran or citrus fiber diet and treated with AOM had a lower incidence (number of animals with tumors) and multiplicity (number of tumors/tumor-bearing animal) of colon tumors and tumors of the small intestine than did those fed the control diet and treated with AOM. Animals fed the diet containing wheat bran developed fewer adenomas and adenocarcinomas of the colon than did the rats fed the control diet; the number of adenomas but not the number of adenocarcinomas was reduced in rats fed the citrus pulp diet. This study thus indicates that diets containing wheat bran and citrus fiber reduced the risk of intestinal cancer.