In vitro genotoxicity assays are extensively used to predict carcinogenic activity in vivo. The standard microbial mutagenicity assays however often fail to yield positive results with mineral oils which are carcinogenic to mice in long-term skin-cancer studies. A comprehensive programme of studies has therefore investigated the basis of this apparently anomalous behaviour. This investigation has addressed the possible effects of oils on the bioactivation of precursor mutagens and the disposition of mutagenic metabolites by studying the microbial mutagenicity of selected precursor mutagens (benzo[a]pyrene, benzo[a]anthracene, 2-aminoanthracene and 2-naphthylamine) and intrinsically reactive mutagens [+/- )-benzo[a]pyrene-4,5-oxide and (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene) in the presence and absence of mineral oils. Notably the mutagenicity associated with the deliberate additions of these mutagens or precursor mutagens to oils was readily detected by the microbial assays. The mutagenicity of only one of the precursor mutagens, benzo[a]pyrene, was significantly reduced by the oils, and then only in the standard plate-incorporation assay. Interestingly the degree of suppression appeared to be related to the polycyclic aromatic hydrocarbon content of the oils. In the case of 2-aminoanthracene large enhancements in its mutagenicity were observed in the presence of oils. These latter findings appear to be due to effects of oils on the bioactivation of precursor mutagens rather than on the disposition of their bioactivation products. The mutagenicity of intrinsically reactive mutagens, of a type generated by bioactivation of polycyclic aromatic hydrocarbons, was not significantly reduced in the presence of mineral oils. This indicates that it is unlikely that components in oils trap or facilitate the deactivation of ultimate mutagens whether these pre-exist in the oil or are formed from precursors by bioactivation in the in vitro test system. Viewed overall these results suggest that mineral oils judged to be carcinogenic on the basis of in vivo studies in mouse skin may possess only very weak genotoxic potential. While this potential is likely to be a prerequisite for carcinogenic action, the current results cause attention to be focussed on other factors, e.g. promotion, as potentially important determinants of the carcinogenic potencies of mineral oils in mouse skin.