We have developed a culture system for detecting and isolating rare hypoxanthine phosphoribosyltransferase-deficient mutants of human epidermal keratinocytes. A thioguanine-resistant variant, 3T3M1, of the Swiss mouse fibroblast line 3T3 was used as a feeder layer to support clonal growth of mutant keratinocytes. A near-diploid, epidermal squamous cell carcinoma line, SCC-13Y, was used as a prototype to determine mutagen treatment conditions, plating density, and phenotypic expression time for maximum mutant recovery. To extend this system to normal keratinocytes, we improved the culture conditions by adding insulin, adenine, and Ham's nutrient mixture F-12, which increased colony-forming efficiencies to 30% in early passage and made feasible the detection of rare mutants in normal epidermal keratinocyte populations. We have quantitated mutation in SCC-13Y and three strains of normal human epidermal keratinocytes after exposure to polycyclic aromatic hydrocarbons, which are activated to their mutagenic forms by cellular mixed-function oxidases. 7,12-Dimethylbenz[a]anthracene and benzo[a]pyrene caused almost no cytotoxicity, but induced thioguanine-resistant mutants at frequencies as much as 50-fold higher than the spontaneous frequency of approximately 10(-6). The mutants were aminopterin-sensitive and possessed no measurable hypoxanthine phosphoribosyltransferase activity; their behavior was indistinguishable from that of keratinocytes cultured from individuals with Lesch-Nyhan syndrome. This mutagenesis assay system should also be applicable to other feeder layer-dependent human epithelial cell types, such as urothelial, mammary, and tracheal epithelial cells.