In a study designed to investigate the effects of dietary synergisms on 1,2-dimethylhydrazine (DMH)-induced rat colon carcinogenesis, fecal pellets were examined for the presence of direct-acting fecal mutagens and levels of Bacteroides fragilis group organisms. Intraperitoneal injections of DMH at 10 mg/kg were given for 16 weeks (weeks 3-18) to 160 male F344 rats consuming 4 supplemental dietary factors in all possible combinations. The dietary factors examined were wheat bran (15%), cholesterol (1%), beef tallow (18%) and indole-3-carbinol (IC) (0.1%). Feces were collected 3, 10, 17, 24 and 31 weeks after commencing the dietary treatments and dichloromethane extracts were assayed using the Salmonella typhimurium TA100 without metabolic activation. The numbers of B. fragilis group organisms were enumerated in feces collected at the same time. Most feces samples were negative for mutagens but extracts from weeks 17-31 showed a significant mutagenic response from the IC factor in the diet. The fecal levels of B. fragilis were significantly increased by the inclusion of cholesterol in the diets. The B. fragilis counts and fecal mutagen production were not correlated (r = 0.09), although species of the B. fragilis group have been implicated in the production on human fecal mutagens.