2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is the most abundant of the heterocyclic amines found in cooked meat. Based on in vitro studies with rats and humans, CYP1A2 is believed to be the primary enzyme responsible for N(2)-hydroxylation, the initial step in the metabolic activation of PhIP. To determine whether CYP1A2 is the primary P450 responsible for metabolic activation of PhIP in mice that leads to tumor formation, neonatal Cyp1a2-null and wild-type mice were treated with approximately 11 (low dose) and approximately 22 (high dose) mg/kg PhIP at days 8 and 15, corresponding cumulatively to 600 and 1200 nmol PhIP, and analyzed at 19-21 months of age. Three major induced tumors were found; lymphomas and tumors in lung and liver. The incidence of lymphoma was higher in Cyp1a2-null females than wild-type females treated with low dose (600 nmol) PhIP whereas no significant differences were observed in other treatment groups of mice. Overall differences in incidences of lung adenoma/adenocarcinoma were in general not consistent among sexes, genotypes and PhIP doses used, although reduced incidences of lung tumors were found in Cyp1a2-null males with low dose (600 nmol) and null females with high dose (1200 nmol) PhIP. Higher incidences of hepatocellular adenoma were observed in Cyp1a2-null female and male mice as compared with wild-type mice. In vitro studies using Cyp1a2-null and wild-type mouse liver microsomes revealed that CYP1A2 is the major enzyme required for PhIP N2-hydroxylation in mouse, the initial metabolic activation of PhIP that is thought to lead to tumor formation. These in vivo and in vitro results suggest that although the metabolic activation of PhIP is carried out primarily by CYP1A2, an unknown pathway unrelated to CYP1A2 appears to be responsible for PhIP carcinogenesis in mouse when examined in the neonatal bioassay. In fact, CYP1A2 may even be protective against all transformation, especially in females.