Weanling female BALB/c mice were continuously fed diets containing combinations of 12 or 24% protein, 4 or 24% fat and 500 ppm of 2 acetylaminofluorene (2‐AAF) for 78 weeks. Mice on the high fat diets demonstrated a significant increase in the incidence of hepatocellular neoplasms and urinary bladder carcinomas. The incidence of both liver tumors and bladder tumors was reduced in the mice fed low fat and low protein diet compared to the other three diets. The protein levels alone appeared to have no effect on the incidence of bladder tumors or liver tumors. Mice fed the diet with lower levels of fat and protein showed a significant decrease in the incidence of bladder carcinomas compared to the mice fed the higher levels of fat andlor protein.