The development of 1,2-dimethylhydrazine (DMH)-induced colon tumors and immune responses were investigated in male BALB/c mice fed six different equicaloric diets. Milk or beef at a low (11%) or high (33%) level supplied the dietary protein, and corn oil (primarily) at a low (5%) or high (30%) level supplied the fat. Eleven weekly injections of DMH (at 20 mg/kg mouse) or saline were administered. At 59 weeks of age, the milk-fed mice had a significantly higher (P less than or equal to .05) colon tumor incidence than the beef-fed mice, 67 and 16%, respectively. Tumor volume and colon weight in the milk-fed mice were also significantly greater. Low natural killer cell activity against [125I]5-iodo-2'-deoxyuridine-labeled colon tumor cells and high serum blocking of antitumor cell activity were observed in the milk-fed-mice. These mice also exhibited higher T-lymphocyte cytotoxicity against colon tumor cells. These results differ from those of our previous studies and those of numerous epidemiologic investigations.