The multifactorial nature of carcinogenesis in man has impelled us to study the effects of various chemicals and conditions in combination. In the present investigation, we examined the effects of low doses of 2-amino-3,8-dimethyl-imidazo[4,5-f]quinoxaline (MeIQx) in combination with carbon tetrachloride (CCl4) on the formation of glutathione S-transferase placental form (GST-P)-positive foci in rat liver. Administration of diet containing MeIQx at 0.4, 4 or 40 p.p.m., representing one-thousandth, one-hundredth and one-tenth of the dose proved to induce hepatocellular carcinomas (400 p.p.m.), for 8 or 12 weeks did not induce GST-P-positive foci. However, 40 p.p.m. of MeIQx when co-administered with CCl4 (0.7 ml/kg, s.c. twice a week) induced preneoplastic lesions: 7- and 3-fold increases in the numbers and areas of GST-P positive foci in week 8, and 8- and 6-fold increases respectively in week 12, over those with CCl4 alone. Furthermore, a marked increase in the number of hyperplastic nodules was observed in this group of rats in week 12. No significant increases of GST-P-positive foci were observed at doses of 0.4 or 4 p.p.m. MeIQx in combination with CCl4. Thus, it is predicted that chronic exposure to 40 p.p.m. of MeIQx eventually results in induction of hepatocellular carcinomas in injured rat liver.