Damage to the human adenomatous polyposis coli (APC) gene is responsible for not only familial adenomatous polyposis but also many sporadic cancers of the entire digestive tract. Using homologous recombination in embryonic stem cells, we recently constructed gene knockout mice with a truncation mutation in the Apc gene. These heterozygous mice developed intestinal polyps. We found that all microadenomas dissected from the earliest polyps had already lost the wild-type allele, indicating loss of heterozygosity (LOH) (Oshima e t al., Proc. Natl. Acad. Sci. USA 92:44824486, 1995). Using these knockout mice, we investigated the effects of 2amino-I -rnethyl-6-phenylimidazo[4,5-b]pyridine (PhlP), one of the most abundant heterocyclic amines found in cooked meat and fish. When PhlP was fed t o these mice a t 400 ppm for 8 wk, the polyp distribution shifted to a larger size range, although the total polyp number did not change significantly. Similar, but weaker, effects were observed with the other heterocyclic amines 2-amino-3-methylimidazo[4,5-flquinoline and 2amino-3,8-dimethylimidazo[4,5-flquinoxaline. On the other hand, intraperitoneal injections of 2-hydroxyamino-I-methyl-6-phenylimidazo[4,5-b]pyridine (N-OH-PhlP) at a higher dose (50 mg/kg) for five consecutive days increased the polyp number significantly. This increment was not associated with mutations in the Apc gene; however, most polyps showed loss of the full-length Apc allele (LOH). These results suggest that PhlP affects intestinal polyp development by accelerating the growth rate of microadenomas. It is also possible that high doses of N-OH-PhlP increase the frequency of Apc gene LOH.