Ultrafine particulate pollutants induce oxidative stress and mitochondrial damage. This study investigated the differences in the size and composition of coarse (2.5–10 μm), fine (<2.5 μm), and ultrafine (<0.1 μm) particulate matter (PM) in relation to their uptake in macrophages and epithelial cells and their ability to induce oxidative stress. The study collected PM from the Los Angeles basin and analyzed their chemical composition, reactive oxygen species (ROS) generation, and ability to induce oxidative stress in macrophages and epithelial cells. UFPs were most potent in inducing cellular heme oxygenase-1 (HO-1) expression and depleting intracellular glutathione. HO-1 expression is a sensitive marker for oxidative stress and is directly correlated with the high organic carbon and polycyclic aromatic hydrocarbon (PAH) content of UFPs. The dithiothreitol (DTT) assay, a quantitative measure of in vitro ROS formation, was correlated with PAH content and HO-1 expression. UFPs also had the highest ROS activity in the DTT assay. Because of their small size, UFPs can penetrate tissues more easily and were studied using electron microscopy to examine subcellular localization. UFPs and, to a lesser extent, fine particles localized in mitochondria, where they induced structural damage, which may contribute to oxidative stress. The study demonstrated that the increased biological potency of UFPs is related to the content of redox cycling organic chemicals and their ability to damage mitochondria. The findings suggest that UFPs are more potent than fine and coarse PM in inducing oxidative stress, as measured by the DTT, HO-1, and glutathione assays. Electron microscopy also indicated subcellular penetration and mitochondrial damage by UFPs and, to a lesser extent, fine particles. The findings correlate with PM organic carbon and PAH composition, suggesting a role of organic agents in generating redox activity. The results from the DTT assay indicate UFPs are capable of producing greater ROS on a microgram basis than fine and coarse particles. This is the first time a quantitative assay has been used to directly measure ROS generation by concentrated ambient particles. UFPs contain a higher percentage of organic carbon than fine and coarse particles, and this has relevance to the biological potency of these particles. The enhanced biological potency of UFPs is directly correlated with the PAH content. Although PAHs are capable of inducing ROS production in macrophages, it is also possible that these compounds may be a surrogate for other redox cycling chemicals in the DTT assay. These data are in accordance with the growing awareness that oxidative stress plays a key role in the induction of airway inflammation. The biological significance of HO-1 expression in the lung is the antioxidant effect of its catabolic product, bilirubin. In the process of hUltrafine particulate pollutants induce oxidative stress and mitochondrial damage. This study investigated the differences in the size and composition of coarse (2.5–10 μm), fine (<2.5 μm), and ultrafine (<0.1 μm) particulate matter (PM) in relation to their uptake in macrophages and epithelial cells and their ability to induce oxidative stress. The study collected PM from the Los Angeles basin and analyzed their chemical composition, reactive oxygen species (ROS) generation, and ability to induce oxidative stress in macrophages and epithelial cells. UFPs were most potent in inducing cellular heme oxygenase-1 (HO-1) expression and depleting intracellular glutathione. HO-1 expression is a sensitive marker for oxidative stress and is directly correlated with the high organic carbon and polycyclic aromatic hydrocarbon (PAH) content of UFPs. The dithiothreitol (DTT) assay, a quantitative measure of in vitro ROS formation, was correlated with PAH content and HO-1 expression. UFPs also had the highest ROS activity in the DTT assay. Because of their small size, UFPs can penetrate tissues more easily and were studied using electron microscopy to examine subcellular localization. UFPs and, to a lesser extent, fine particles localized in mitochondria, where they induced structural damage, which may contribute to oxidative stress. The study demonstrated that the increased biological potency of UFPs is related to the content of redox cycling organic chemicals and their ability to damage mitochondria. The findings suggest that UFPs are more potent than fine and coarse PM in inducing oxidative stress, as measured by the DTT, HO-1, and glutathione assays. Electron microscopy also indicated subcellular penetration and mitochondrial damage by UFPs and, to a lesser extent, fine particles. The findings correlate with PM organic carbon and PAH composition, suggesting a role of organic agents in generating redox activity. The results from the DTT assay indicate UFPs are capable of producing greater ROS on a microgram basis than fine and coarse particles. This is the first time a quantitative assay has been used to directly measure ROS generation by concentrated ambient particles. UFPs contain a higher percentage of organic carbon than fine and coarse particles, and this has relevance to the biological potency of these particles. The enhanced biological potency of UFPs is directly correlated with the PAH content. Although PAHs are capable of inducing ROS production in macrophages, it is also possible that these compounds may be a surrogate for other redox cycling chemicals in the DTT assay. These data are in accordance with the growing awareness that oxidative stress plays a key role in the induction of airway inflammation. The biological significance of HO-1 expression in the lung is the antioxidant effect of its catabolic product, bilirubin. In the process of h