Synopsis The mutagenicity of aniline was investigated in association with norharman using Aroclor 1254-induced and uninduced liver S9 fractions. The non-mutagenicity of aniline in the Salmonella/mammalian microsome test was also re-examined under various experimental conditions. Aniline caused no increase in the number of revertants per plate in Salmonella typhimurium strains TA1535, TA100, TA1537, TA1538 and TA98 at concentrations of up to 5000 mug/plate. S9 preparations from rat liver, hamster liver, rat spleen, or hamster spleen were all ineffective in activating aniline to a mutagen in tests using various concentrations of S9 in S9 mix. In contrast to the results with aniline alone, mixtures of aniline and norharman were mutagenic in strain TA1538 in the presence of S9 fractions from liver of rats pretreated with Aroclor 1254. The observed mutagenicity decreased as a function of the concentration of S9 in S9 mix. In the presence of uninduced liver S9 preparations, the mutagenicity of the mixture of the two compounds was slight (three-fold).