Cadmium is a toxic environmental contaminant that is carcinogenic in humans and rodents. Although cadmium has proven to be mutagenic in a variety of assay systems, exactly how cadmium achieves gentoxicity is poorly understood. To define the mechanism(s) underlying the mutagenicity and comutagenicity of cadmium, human Ad293 cells were exposed to subtoxic doses of the metal and transfected with untreated or anti-5-methylchrysene-3,4-dihydrodiol 1,2-epoxide (5-MCDE)-treated pS189 shuttle vector. Alterations in the frequency, types, and distribution of mutations were subsequently assessed in the supF gene of pS189 that was replicated in Ad293 cells and screened in indicator bacteria. Doses of 0.5 and 1 microM CdCl2 increased the mutation frequency of untreated pS189 by approximately 4- and 8-fold, respectively, with no apparent effect on the types of mutations generated. In contrast, host-cell exposure to cadmium had little or no effect on the frequency, types, or distribution of mutations generated with 5-MCDE-treated pS189. These results indicate that cadmium increases mutagenesis of untreated pS189 by affecting a process that is not involved in mutagenesis of the 5-MCDE-treated vector. Although it is not clear exactly how host-cell exposure to cadmium increases background mutagenesis, presumably, the mutagenic effect does not involve cadmium interaction with the cellular machinery used to replicate past bulky DNA lesions.