The effect of dietary undegraded carrageenan (Viscarin 402) on colon carcinogenesis was studied in female inbred F344 rats. Weanling rats were fed semipurified diets containing 0 or 15% undegraded carrageenan. At 7 weeks of age, all animals except controls were given azoxymethane (AOM) s.c. at a dose rate of 8 mg/kg body weight per week for 10 weeks or methylnitrosourea (MNU) intrarectally at a dose level of 2 mg/rat twice a week for 3 weeks. The AOM groups were autopsied 40 weeks and the MNU groups 30 weeks after the first injection. No tumors were induced in the colon or in other organs of untreated rats fed the control diet. One untreated rat fed the carrageenan diet showed a colon adenoma. The animals fed the carrageenan diet and treated with AOM or MNU had a higher incidence of colorectal tumors (number of rats with colorectal tumors and number of tumors per tumor-bearing rat) than did those fed the control diet and treated similarly. The undegraded carrageenan (Viscarin 402) in the diet had an enhancing effect in colorectal carcinogenesis in rats evoked by AOM or MNU.