The Ah locus represents a complex "cluster" of genese controlling the induction of numerous drug-metabolizing enzyme "activities" by polycyclic aromatic compounds. Allelic differences at the Ah locus are reflected in the large differences in inducibility of cytochrome P1-450 and benzo[a]pyrene metabolism in numerous tissues when the mice receive the chemical daily in their diet. This experimental model system offers to the hematologist and clinical pharmacologist a means to study genetic differences in toxic chemical depression of the bone marrow, as well as a potential model to study aplastic anemia and leukemia explainable on a single-gene basis. The genetically "responsive" individual who is at increased risk for cancer caused by subcutaneous or topical or intratracheal polycyclic hydrocarbons is at decreased risk for toxicity of the bone marrow and leukemia caused by oral benzo[a]pyrene (when compared with the genetically "nonresponsive" individual receiving the same dose of the same xenobiotic). In other words, tissue sites in direct contact with the carcinogen develop cancer in responsive animals because of induced P1-450; tissues in distant sites of the body may develop malignancy in nonresponsive animals because more carcinogen reaches that tissue due to decreased P1-450 induction all over the body and therefore decreased detoxication. Not only the dct with the carcinogen develop cancer in responsive animals because of induced P1-450; tissues in distant sites of the body may develop malignancy in nonresponsive animals because more carcinogen reaches that tissue due to decreased P1-450 induction all over the body and therefore decreased detoxication. Not only the dct with the carcinogen develop cancer in responsive animals because of induced P1-450; tissues in distant sites of the body may develop malignancy in nonresponsive animals because more carcinogen reaches that tissue due to decreased P1-450 induction all over the body and therefore decreased detoxication. Not only the dose but the route of administration and the tissue in which the malignancy or toxicity develops are therefore very important in the interpretation of data from tumorigenesis or toxicity experiments involving P1-450 inducers such as polycyclic hydrocarbons. There exists sufficient evidence that heritable variation of the Ah locus occurs in man. Growing evidence indicates that persons with higher aryl hydrocarbon hydroxylase inducibility in their cultured mitogen-activated lymphocytes may have a statistically significantly increased risk for certain types of cancer and drug toxicity. It remains to be determined at the present time, however, whether this genotype can be used as a biochemical marker in the individual patient for predicting increased susceptibility to certain types of environmentally caused cancers or toxicity in man.