The purpose of the present study was to investigate whether 3 different types of dietary fiber, wheat bran, carrot fiber and citrus pectin, influenced the induction of colorectal tumors produced by 1,2-dimethylhydrazine in rats. In all groups, the tumor yield was high (87-97%). In the wheat bran and carrot fiber groups, the incidence of colorectal tumors was not significantly different from that of the group fed on the fiber-free basic diet. The citrus pectin group had a significantly higher incidence of colorectal tumors (P < 0.001). An increased number of auditory duct tumors was also noted in this group. In a separate experiment, dietary pectin induced a 10-fold increase in fecal .beta.-glucuronidase activity but did not alter this activity in the bowel wall. Dietary fiber may protect against the induction of colorectal tumors, but this was not the case in this experiment. It is possible that the high tumor yield made the demonstration of a weak protective effect of wheat bran impossible. The reason for the increased occurrence of tumors in the citrus pectin group is obscure and will be subjected to further investigation. Fecal .beta.-glucuronidase activity might be 1 factor of importance in the activation of the carcinogen.