The Ah receptor is a soluble protein complex that mediates carcinogenesis by a wide range of environmental pollutants, including polycyclic aromatic hydrocarbons, heterocyclic amines, and polychlorinated aromatic compounds. The best understood activity of the receptor concerns its role in the induction of cytochrome P450IA1. We undertook a somatic cell genetic analysis of P450IA1 induction using the mouse hepatoma cell line, Hepa-1. Clones of Hepa-1 were isolated that are defective in induction of P450IA1. Evidence was obtained that the clones are mutational in origin. Cell fusion experiments demonstrated that a few of the mutants are dominant, while the majority are recessive. The dominant mutants were shown to synthesize a repressor of P450IA1 transcription. The recessive mutants were assigned to 4 complementation groups (probably corresponding to 4 different genes). Complementation group A corresponds to the P450IA1 structural gene. Mutations in the B, C and D genes all affect functioning of the Ah receptor. A 'reverse selection procedure', whereby cells that express P450IA1 inducibility can be selected from a majority population of cells lacking inducibility, was developed. The reverse selection procedure was used to isolate transfectants of representative recessive mutants in which the mutational defects are complemented by exogenously applied genomic DNA. A human DNA-derived transfectant of a C- mutant was used to clone the human C gene. The C gene is not the ligand-binding subunit of the Ah receptor but is a protein that is required for translocation of Ah receptor-ligand complexes from cytoplasm to nucleus. In analogous experiments the dominant gene from one of the dominant mutants was transfected into wild-type Hepa-1 cells. Success in transfecting the dominant gene should provide the means for cloning it.