Nitropyrenes cause frameshift mutations in Salmonella typhimurium. This activity which is restricted to frameshift mutations is unusual in several respects: (a) Nitropyrenes, as a class, are the most mutagenic chemicals reported in the literature; (b) The mutagenicity depends upon the formation of adducts between DNA and nitropyrene metabolites; (c) The penultimate intermediates responsible for mutagenic activity (hydroxylamines) are not obtained in all instances by reduction of the nitro function by the "classical" nitroreductase (the one that acts on nitrofurans and other simple nitrated polycyclic aromatic hydrocarbons) but by another nitroreductase which appears to be specific for higher nitrated polycyclic aromatic hydrocarbons; (d) The mutagenicity of nitropyrenes is enhanced when resting rather than growing bacterial cultures are used.