A human teratoma cell culture system that can be used to detect a variety of chemical mutagens was described. The system used an epithelial cell line, designated P3, which was cloned from a human teratoma cell line. The induction of mutations at the hypoxanthine- guanine-phosphoribosyl-transferase (HGPRT) locus, situated on the X- chromosome, rendered the cells resistant to the nucleic acid analogue 6-thioguanine (thioguanine). The loss of HGPRT enzyme activity could arise from a variety of genetic insults at this locus, including frameshift mutations, base pair substitutions, chromosomal aberrations, and deletions. The mutability of the P3 cells for thioguanine resistance was evaluated using benz(a)pyrene (50328) (BaP) and N-methyl-N'-nitro-N-nitrosoguanidine (70257) (MNNG). The cloning efficiency of the P3 cells was 69 percent, whereas the cloning efficiency of the mutant cells ranged from 45 to 75 percent. The effects of MNNG or BaP on HGPRT activity in P3 cells and clones were determined. HGPRT enzyme activity was reduced by more than 10 fold in the clones, compared to P3 cells. Thioguanine resistant P3 cells were activated with human BJ cells and incubated with benzo(e)pyrene (192972) (BeP), pyrene (129000), chrysene (218019), BaP, 3-methylcholanthrene (56495) (MCA), or 7,12- dimethylbenz(a)anthracene (57976) (DMBA) and assayed for the induction of thioguanine resistant mutants. DMBA, MCA, BaP, and chrysene induced thioguanine resistant mutants, the degree of induction being directly related to their carcinogenic potency. Pyrene and BeP were inactive or exhibited a limited induction of thioguanine resistant mutants. The authors conclude that the human P3 epithelial cell mutagenesis assay can detect genetic damage induced by physical agents and direct acting and proximate mutagens or carcinogens.