Mutagens in the semivolatile phase of airborne particulate matter, diesel and gasoline engine emissions were investigated using chemical and biological assays. The previously unknown mutagens, 1-(3-)nitro-6-azabenzo[a]pyrenes (1-N-6-ABP and 3-N-6-ABP) and 1-(3-)nitro-6-azabenzo[a]pyrene-N-oxides (1-N-6-ABPO and 3-N-6-ABPO), were detected in the semivolatile phase, which was adsorbed onto XAD-4 resin combined with a Teflon-coated fibre filter. Dichloromethane extracts of materials adsorbed onto XAD-4 resin were divided into acidic, neutral and basic by liquid-liquid separation. These chemicals in the basic fraction were the major mutagens in the semivolatile phase when they were bioassayed using Salmonella typhimurium TA98 in the absence of S9 mix; the mutagenicity of the basic fraction contributed at the rate of 42.9-68.8% of that of crude extracts for airborne, diesel and gasoline emission materials. As the mutagens were found to be difficult to analyse by gas chromatography (because of their probable absorption onto the packing) they were purified by HPLC and analysed by mass spectrometry. The concentrations of 1-N-6-ABP, 3-N-6-ABP, 1-N-6-ABPO and 3-N-6-ABPO detected were 1.1, 1.2, 0.8 and 0.3 ng/g, respectively, of materials in the airborne sample, and 4.9, 7.7, 2.2 and 3.8 ng/g, respectively, of materials in the diesel emission. Only 1-N-6-ABP (3.4 ng/g) and 3-N-6-ABP (4.9 ng/g) were detected in the semivolatile phase of the gasoline engine emission.(ABSTRACT TRUNCATED AT 250 WORDS)