The pro-mutagenicity of chemically-induced methylation of DNA at the O6 position of deoxyguanosine was studied in cultured adult rat liver epithelial cells. To modify the level of O6-methyldeoxyguanosine (O6-medGuo) resulting from exposure to an alkylating agent, partial depletion of the O6-alkylguanine-DNA alkyltransferase (AGT) repair system was produced by pretreatment of ARL 18 cells with a non-toxic dose of exogenous O6-methylguanine (O6-meG). Exposure of cells to 0.6 mM O6-meG for 4 h depleted AGT activity by about 40%. Intact and pretreated cells were exposed to a range of doses of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), and mutagenesis at the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus was quantified by measurement of 6-thioguanine-resistant mutants. The mutagenicity of MNNG was dose dependent and was greater in O6-meG pretreated cultures than in intact cultures. Immunoslot blot measurement of O6-medGuo employing a mouse monoclonal antibody demonstrated that MNNG produced O6-medGuo and that the intact liver cells were efficient in eliminating this lesion from their DNA. Since depletion of AGT would be expected to affect the rate of elimination of only O6-medGuo, it is concluded that this lesion is highly pro-mutagenic.