The protective role of dietary fibers on tumorigenic effects of 1,2-dimethylhydrazine (DMH) on the rat colon was studied using histochemical, immunohistochemical, and biochemical analyses. Rats were injected with DMH (20 mg/kg sc) for five weeks, once a week, and were fed laboratory chow (Control I), semisynthetic fiber-free diet (Control II), or 18% or 25% assorted fiber (cellulose, beans, corn) diet. The rats were sacrificed 12, 16, and 24 weeks after the carcinogenic injections. Adenomatous tumors developed in 90-100% of the animals fed chow or fiber-free diets. A diet with high concentrations (25%) of corn dietary fiber significantly decreased the tumor incidence (40% and 42%) and tumor yield (p < 0.01). Diets with lower concentrations of fibers (18%) did not protect against tumorigenic effects of DMH. No differences were found in pathological parameters of tumors obtained from different dietary groups: the damage index (percentage of damaged cells per section) was very similar in all groups studied. Diets enriched with dietary fibers (25%) had a significant protective effect in DMH-induced rat colon cancer.