In vitro studies with the organic extracts of diesel particles have suggested that hydrocarbons such as PAH may play a role in an inflammatory response, but these have been limited by the possible artifacts introduced in the particle collection and processing. In this study, we avoid these artifacts and use an activated carbon denuder to remove hydrocarbons from the exhaust stream to investigate their role in the inflammatory response. Human bronchial epithelial cells (16HBE14o) were exposed at the air-cell interface to diluted and aged exhaust from a diesel generator operated at partial and no load conditions. When particles were removed with a filter before cell exposure, exhaust gases accounted for almost half of the response compared to the whole exhaust. Removal of gas phase and a portion of the particle phase hydrocarbons with the denuder decreased the interleukin-8 (IL-8) secretion to unexposed levels.