Linear alkanes of specific chain length between 6 and 16 carbon atoms, an aryl derivative of dodecane, and a phorbol diester were tested in a cell culture system for relative ability to enhance mutagenesis induced by a chemical carcinogen, methylazoxymethanol acetate (MAM). Mutation frequencies at the ouabain-resistance locus were measured. Results indicated an excellent correlation between the relative activities of the above compounds in enhancing mutagenesis in the in vitro culture system and their tumor-promoting activities in mouse skin. None of the compounds tested showed mutagenic activity per se, further lending support to the theory that promoters act via derepression of latent carcinogen-induced damage to the genome.