The authors sought to detect S9-activated mutagens in diesel-exhaust particle extracts by fractionating extracts using high-performance liquid chromatography (HPLC) and carrying out Ames assays (strains TA98 and TA100) with and without S9 mix on the eight fractions obtained thereby. S9 activation of the fraction containing unsubstituted and alkyl-substituted polycyclic aromatic hydrocarbons (PAH) was at the limits of detectability for strain TA100 and undetectable for TA98. There was only one fraction that showed S9 activation; it was 20-fold more active with S9 mix. Six other fractions were strongly direct-acting and showed no enhancement of activity with S9 mix. The S9-activated mutagens detected when concentrated in a chromatographic fraction could not be detected in unfractionated extracts because the direct-acting mutagenicity of the other components dominated. They were unable to identify the indirect-acting mutagens from GC-MS data.