Induction of 6-thioguanine (TG) resistance by chemical mutagens was examined in a line of cells derived from a human epithelial teratocarcinoma cell clone. The cells, designated as P3 cells, have a stable diploid karyotype with 46(XX) chromosomes, including a translocation between chromosomes 15 and 20. Efficient recovery of TG-resistant mutants induced by the direct-acting mutagens: N-methyl-N'-nitro-N-nitrosoguanidine (MNNG); 7beta,8alpha-dihydroxy-9alpha,10alpha-epoxy-7,8,9,10-tetrahydrobenzo(alpha)pyrene (BPDE); and benzo(alpha)pyrene (B(alpha)P); activated in a cell-mediated assay, required an expression time of 7 days and a saturation density of 2X10 to the fourth power cells/60-mm petri dish. The TG-resistant mutant cells induced by MNNG and BPDE maintained their resistant phenotype 4-6 weeks after isolation. This mutant phenotype was associated with a morre than 10-fold reduction in hypoxanthine-guanine phosphoribosyl transferase (HGPRT) activity relative to that of the parental P3 cell line, which was shown to catalyze the formation of 4.6 pmoles inosine-5'-monophosphate (IMP)/min/micrograms protein. Induction of TG resistance was also observed in P3 cells cocultivated in a cell-mediated assay with human breast carcinoma cells. Which are capable of polycyclic aromatic hydrocarbon (PAH) metabolism.