Immunomodulatory effects of chemicals adsorbed to particles with aerodynamic diameter below 0.49 microm (PM(0.5)) collected in winter 2001 at three sampling points (industrial area [LPIn], traffic-influenced urban area [LPCi], and control area [LPCo]) of La Plata, Argentina, were investigated. The sampling of particulate matter was carried out with high-volume collectors using cascade impactors. PM(0.5)-adsorbed compounds were hexane-extracted by accelerated solvent extraction. For immunological investigations, human peripheral blood lymphocytes were activated by phytohemagglutinin and exposed to dimethyl-sulfoxide dilutions of PM(0.5)-extracts for 24h. Vitality/proliferation was quantified using MTT, released interferon-gamma (IFN-gamma) and interleukin-4 (IL-4) by ELISA. Cytokine production but not vitality/proliferation was significantly suppressed by all of the highest extract concentrations. Generally, suppression of IFN-gamma by PM(0.5)-extracts was stronger than those of IL-4. Based on administered mass of PM(0.5), all extracts suppressed IFN-gamma production nearly uniform. Contrary, LPCi-extracts exerted maximum IFN-gamma suppression based either on air volume or regarding PM(0.5)-adsorbed PAH. Also the ranking of PM(0.5)-associated effects on IL-4 production differs in dependence of the chosen reference points, either mass or [microg/ml] or air volume [m(3)/ml] related dust quantities in cell culture. Based on the corresponding air volume, LPCi-extracts inhibited IL-4 production to the maximum extend, whereas suppression of IL-4 was comparable based on concentrations. This indicates that not only the disparate PM(0.5)-masses in air cause varying impacts, but also that disparities in PM(0.5)-adsorbed chemicals provoke different effects on immune responses and shifts in the regulatory balance that might have implications for allergy and cancer development.