We evaluated the preventive effects of wheat bran (40%, weight percent of diet) on the development of large bowel carcinoma induced by weekly intraperitoneal administration of 25 mg/kg (body weight) of 1,2-dimethylhydrazine over 19 weeks in the rat. Feces number and volume per pellet in wheat bran diet group were significantly greater than those in basal diet group, and the induction rates of tumor in the wheat bran diet group was found to be significantly lower than that in the basal diet group. These results suggested that wheat bran was effective in inhibiting large bowel carcinogenesis, and that the increased fecal weight and volume from wheat bran may be involved in such an inhibitory effect.