The effects of multiple dietary influences on 1,2-dimethylhydrazine [(DMH) CAS: 540-73-8]-induced colon cancer in rats were studied. A 2(4) factorial experimental design was used to examine the main and interactive effects of 15% wheat bran (WB), 1% cholesterol (CH) with cholic acid, 20% beef tallow (BT), and 0.1% indole-3-carbinol (IC) on 160 male F344 rats treated ip with DMH (10 mg/kg) weekly for 16 weeks. The test diets were fed for 3 weeks before, 16 weeks during, and 12 weeks after DMH administration. At necropsy, total weight gain, liver and spleen weights, serum CH levels, liver aryl hydrocarbon hydroxylase (AHH) activity, and the size, number, incidence, and location of intestinal tumors were analyzed for dietary factor effects. The most significant inducer of tumors was the combination of CH + BT + IC acting in synergism. The single main effect most responsible for tumor morbidity was IC, which appeared to enhance tumorigenesis via its role as an inducer of AHH activity. The WB decreased tumor incidence and burden when added to diets also containing CH, but it otherwise increased tumor burden per tumor-bearing animal and incidence in all other diets. This study demonstrated the need for examining synergistic and antagonistic interactions among dietary initiators and/or promoters of colon carcinogenesis, as well as implicating IC as a significant factor in the development of DMH-induced tumors in rats.