Urban particulate matter (UPM) includes particles of size smaller than 10 μm (PM10), which may impact on human respiratory and cardiovascular health. It has been reported previously that PM10 can induce DNA damage. We have collected size-fractionated PM10 at the roadside and measured the induction of DNA damage by different-sized UPM using the alkaline Comet assay and the plasmid strand-break assay. We found that foil disks were more suitable for collecting UPM than quartz fiber filters, as the UPM could be easily extracted from the foil disks and accurately weighed. Using the Comet assay, all size fractions induced DNA damage in A549 lung epithelial cells, with the finer fractions (D50% = 0.65 μm and lower) inducing the most damage. In the plasmid strand-break assay, in which DNA damage is induced by free-radical species generated in solution, the most damage was also induced by the finer fractions, although the finest fraction (D50% < 0.43 μm) did not induce as much damage as D50% = 0.65 and 0.43 μm. When an organic extract of a standard UPM sample was compared to the whole particles and the washed particles in the Comet assay, it was found that around 75% of the damage induced by the whole UPM could be induced by the organic extract. These results show that finer particulates have the greatest ability to induce DNA damage in lung epithelial cells and naked DNA, and that both organic and inorganic components of the UPM contribute to its genotoxic effects. Environ. Mol. Mutagen., 2005. © 2005 Wiley-Liss, Inc.