2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), which is a mutagenic compound present in fried beef and beef extracts, was given orally to CDF1 mice at a concentration of 0.06% in the diet for 84 weeks. Liver tumors were induced in 43% of males and 91% of females fed MeIQx. The incidences of liver tumors in mice of both sexes were significantly higher in groups fed MeIQx than in control groups. The incidences of lung tumors in females fed MeIQx and of lymphomas and leukemias in both sexes fed MeIQx were also significantly higher than in the respective controls.