The dose-dependence of 2-amino-1-methyl-6-phenylimidazo-[4,5-b]pyridine (PhIP) carcinogenicity was investigated in F344 rats of both sexes administered the heterocyclic amine in the diet at concentrations of 25 or 100 p.p.m. for up to 104 weeks. Incidences of mammary adenocarcinomas were 7% (2/30) for 25 p.p.m. and 47% (14 of 30 rats) for 100 p.p.m. in females and those of colon adenocarcinomas were 43% (13/30) for males and 13% (4/30) for females of the 100 p.p.m. groups. No mammary adenocarcinomas were induced in males and no colon carcinomas were observed in the 25 p.p.m. groups of either sex. Furthermore, development of lymphocytic leukemia was apparently enhanced by PhIP in males. In a separate experiment, dose-dependent induction of aberrant crypts in the large intestine, considered as preneoplastic lesions, was evident after 8 weeks feeding of PhIP-supplemented diet at doses of 25, 100 or 400 p.p.m. Thus a clear dose-dependency was demonstrated for both colon and mammary carcinogenesis. Since PhIP is a particularly abundant heterocyclic amine and its carcinogenic organotropism overlaps with the types of neoplasias most commonly observed in western countries, the compound may be extremely important with respect to human cancer development.