The effects of different types of diets on colon carcinogenesis by 1,2-dimethylhydrazine and on the immune system were studied in W/Fu rats. Six different types of diets were used in two sets of experiments. Rats in each group were fed the respective diets immediately upon weaning. 1,2-Dimethylhydrazine dihydrochloride was administered s.c. at a dosage of 15 mg/kg weekly in two divided doses. The rats were followed by sequential laparotomies for the development of gastrointestinal (GI) tumors until death. Tumors appeared earlier, and the total number of GI tumors, particularly those of the colon, was higher in rats fed diet enriched with fat from animal sources. In these rats the GI tumors metastasized more frequently, and their survival, after appearance of the first GI tumor, was significantly shortened. The diet low in animal fat and enriched with carbohydrate reduced the number of GI tumors and delayed their appearance. Semisynthetic elemental diet accelerated the appearance of colon tumors without increasing the total number of GI tumors over the life span of the animals. Serum cholesterol levels evaluated during carcinogenesis suggest a correlation between serum cholesterol levels and the increased frequency of colonic tumors. The alterations in serum immunoglobulin G levels, lymphocyte counts, and surface immunoglobulin-bearing lymphocytes evaluated at different times during carcinogenesis suggested a biphasic ("M type") immune response. Rats fed low residue diets and/or diets containing fat from animal sources had depressed serum immunoglobulin G levels. However, the pattern of immune response was similar in groups of rats fed different types of diets.