The modifying effects of 2, 2'-[(4-aminophenyl) imino] bisethanol sulfate (4APE) on liver carcinogenesis were investigated in male F344/DuCrj rats initially treated with N-nitrosodiethylamine (DEN). Two weeks after a single dose of DEN (200 mg/kg, intraperitoneally), rats were given 4APE at dietary levels of 1, 000, 330 and 110 parts per million (ppm), or 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB) at 600 ppm as a positive control for 6 weeks. At week 3 following DEN administration, all animals were subjected to partial hepatectomy. No adverse effects on survival and body weight were seen in rats treated with 4APE, even at the highest dietary levels. Marked growth retardation and significant increase of relative liver weight were found in animals treated with the known hepatocarcinogen 3'-Me-DAB, which demonstrated marked promoting activity as evidenced by significantly increased values for γ-glutamyl transpeptidase (γ-GT) positive foci as compared with the controls given DEN alone. In contrast, 4APE did not significantly increase the level of γ-GT positive foci over that induced by DEN initiation alone. From these results, it is concluded that 4APE does not possess promoting activity for liver carcinogenesis.