The effects of a tumor-promoting agent on the frequency of mutation to ouabain resistance and survival of Chinese hamster cells treated with a chemical carcinogen have been investigated. 12-O-Tetradecanoyl-phorbol-13-acetate (TPA) significantly enhanced the mutation frequency induced by the carcinogen, methylazoxymethanol acetate (MAM), without having similar effects on cytotoxicity, at concentrations of 2 micrograms/ml or less. The observed degree of enhancement of mutagenesis increased with promoter concentration up to the point where the latter exhibited frank toxicity. Exposure of the cells to the promoter for a period of 2 or 6 h was found ineffective, but subsequently a significant enhancement was found after a 27-h exposure time. The maximum effect occurred after a 5-day exposure, with a increase in the mutation frequency of 250%. Treatment of cells with TPA alone resulted in no enhancement of spontaneous mutation rates, nor did treatment of cells prior to addition of carcinogen-induced mutagenesis. In contrast, TPA was found to be effective when applied as late as 6 weeks following carcinogen treatment. These results are consistent with the hypothesis that TPA owes its promoting activity toward chemically-induced mutagenesis and carcinogenesis to its ability to enhance expression of latent somatic genetic modifications by epigenetic mechanisms. They do not support mechanisms involving TPA-induced inhibition of DNA-repair replication, or mutagenic activity of TPA per se. The notably similar qualitative response to TPA of several parameters in mouse-skin tumorigenesis and Chinese hamster cell mutagenesis suggest that the mechanism of action of the promoter is similar in the 2 diverse biological systems.