Diesel exhaust particles are major constituents of ambient air pollution, and are associated with respiratory and cardiovascular diseases and lung cancer. The organic part of the particles is heterogenic and complex, and seems to be responsible for many of the adverse effects. Increased formation of ROS is often connected to the adverse effects. We have therefore investigated the effect of an organic extract of diesel exhaust particles on the reactive oxygen species (ROS) status in human neutrophil granulocytes and rat alveolar macrophages in vitro. ROS formation were studied by three different assays namely the use of DCFH-DA, lucigenin and luminol. The organic extract increased ROS assayed with DCFH-DA, but it decreased the amount of ROS in cells stimulated by PMA in all three assays. The identities of the ROS affected were further studied in cell free systems. The cell free studies confirmed that the extract had scavenging effects against superoxide, hypochlorite and to a smaller extent against peroxynitrite, but not against the hydroxyl radical and nitric oxide. ROS take part in the intracellular signalling pathways as well as in the defence against invading microorganisms, and the possible effects of interference of the redox status in the cells are discussed.