Diesel exhaust particulates (DEPs) can constitute a large component of the particulate air pollution in urban areas and is a health concern. The effects of DEP on nitric oxide (NO) production by a murine macrophage cell line (RAW264.7) in response to interferon-gamma (INFgamma), lipopolysaccharide, (LPS) and Bacillus Calmette-Guerin (BCG) were studied. The DEP was fractionated into organic and inorganic fractions (carbonaceous core). The organic portion was further divided into asphaltene, saturates, less polar aromatics, more polar aromatics and resins-containing fractions. Each fraction was tested for the ability to suppress NO production from BCG-stimulated macrophages. DEP crude organic extract, more polar aromatic hydrocarbon, and resin fractions dose-dependently inhibited BCG-stimulated NO production. It is concluded that the responsiveness of the macrophages to stimuli, such as BCG, is suppressed by DEP and that this activity is most predominant in the polar aromatic hydrocarbons and resins-containing fractions.