Nitro-substituted polycyclic aromatic hydrocarbons are widespread environmental contaminants that are mutagenic and potentially carcinogenic. 3-Nitrofluoranthene (3-NFA) has been found in diesel and ambient air particulate extracts; although less abundant than its isomer 1-nitropyrene (1-NP), its content in these extracts correlates well with their direct-acting mutagenic potency in the Ames assay. 3-NFA is more mutagenic towards Salmonella TA98 than 1-NP both without (2 500 His+rev/nmol vs. 350 rev/nmol) and with exogenous metabolic activation (250 vs 60 rev/nmol). 3-Amino-fluoranthene was less active than 3-NFA both with and without S9 (100 and 40 rev/nmol respectively); 3-acetamidofluoranthene was even less active with S9 (60 rev/nmol), and quite inactive without S9. In contrast, 1-acetamidopyrene (400 rev/nmol +S9) was more active than the parent 1-NP. The activity of 3-NFA was substantially reduced in the nitroreductase-deficient strains of TA98 (TA98NR and TA98/1,8-DNP6), indicating that the 'classical' nitroreductase and the transacetylase enzymes are involved in activation of 3-NFA.