Nitropyrenes are a group of widespread environmental pollutants, some of which are highly potent as bacterial and mammalian cell mutagens and as animal carcinogens. A quantitative bacterial forward mutation assay, based on resistance to 8-azaguanine (8-AG) in Salmonella typhimurium TM677, was employed as an alternative to reversion assays to reexamine the mutagenicity of 1-, 2-, and 4-nitropyrene (1-, 2-, and 4-NP) and 1,3-, 1,6-, and 1,8-dinitropyrene (1,3-, 1,6-, and 1,8-DNP) in the presence and absence of rat liver postmitochondrial supernatant (PMS). The major finding is that 2-NP, reported as a potent mutagen in the absence of PMS in bacterial reversion assays, was inactive in the absence of PMS in this assay. However, 2-NP was mutagenic in the presence of PMS. The implications of this observation with respect to sample purity and the metabolism of 2-NP are discussed. Without PMS the following minimum detectable mutagen concentration (MDMC) potency series expressed as nmol/ml was obtained: 1,8-DNP (0.5 x 10(-3)), 1,6-DNP (1.2 x 10(-3)), 1,3-DNP (2.3 x 10(-3)), 4-NP (0.2), 1-NP (0.2), 2-NP (> 1200), pyrene (> 1500). With PMS the potency series was: 1,6-DNP (0.7), 1,8-DNP (2.1), 4-NP (2.2), 2-NP (2.6), 1,3-DNP (3.7), 1-NP (4.6), pyrene (> 1500). With the exception of 2-NP, all the nitropyrenes were more mutagenic without PMS than with PMS. The greatest difference was observed with the dinitropyrenes, which were three orders of magnitude less potent in the presence of PMS. Pyrene, often reported as a bacterial mutagen in the presence of PMS, was nonmutagenic in this assay when a purified sample was tested.