1. The metabolism of 7,12-dimethylbenz[a]anthracene (DMBA) by rat, hamster and rabbit fibroblasts has been studied and compared with the metabolism of the hydrocarbon by liver homogenates. The metabolsim of DMBA by cell cultures and by liver homogenates was very similar. The ethyl acetate-soluble metabolites identified included dihydrodiols, phenols and hydroxymethyl derivatives. Other more polar metabolites and unidentified water-soluble metabolites were also formed. 2. High yields of phenols and other more polar metabolites, which may be tetrahydrotetrols, were produced by rabbit fibroblasts. 3. Kinetic studies showed that the metabolic activity of rabbit fibroblasts was high but that the conversion of DMBA into water-soluble metabolites was lower than with hamster and rat fibroblasts. 4. 7,12-Dimethylbenz[a]anthracene-induced cytotoxicity was inversely related to the conversion of metabolized DMBA to water-soluble derivatives.