Abstract In order to investigate the chemical nature and diversity of the mutagens in ambient airborne particles (AAP) and coal fly ash (CFA), extracts of these particles were subjected to bioassay-directed fractionation. Open-column as well as high-pressure liquid chromatography were used as fractionation techniques after purification by means of liquid-liquid partition. Extracts and fractions were tested with the Salmonella/microsome test using various tester strains, among them TA98NR and analysed for polycyclic aromatic hydrocarbons (PAH) and some nitrated PAH. Seven distinct groups of mutagens could be discerned in AAP. Two of them were identified as PAH and mono-nitro PAH; the first accounted for about 5% of the total effect in TA98 with S9 and about 20% of the total effect in TA100 with S9. For the mono-nitro PAH, these percentages were about 12, 24, 14 and 13 for TA98 with and without S9 and TA100 with and without S9, respectively. A significant contribution of dinitropyrenes (possibly together with other multiply nitrated PAH) could be ruled out. Five more polar groups of mutagens could be isolated in AAP, which showed clearly nitroreductase-dependent direct mutagenicity. Besides direct-acting nitrocompounds, these groups also comprised indirect acting compounds (S9 depenent); possibly, but not necessarily, these are also nitro-compounds. The distribution of the mutagenicity over the various active chromatographical fractions was clearly strain and S9 dependent. The use of strains TA100 and TA97 pointed to the importance of indirect mutagens, also for the more polar fractions. PAH did not significantly contribute to the mutagenicity of CFA. These particles contained two groups of mutagens that were not found in AAP: a group, the effect of which is most probably caused by dinitropyrenes and comparable compounds and a polar group containing nitro-reductase dependent direct mutagens.