Respirable particulate matter generated during incomplete combustion of fossil fuels may principally target the cells found in the distal region of the lung. This study characterizes some of the effects that a model particulate matter has on the induction of heme oxygenase (HO)-1 in macrophages. HO-1 is a highly inducible stress response gene that has been demonstrated to modulate chemical, physical, and environmental stimuli. Cultured macrophages (RAW 264.7 cells) exposed continuously to a well-defined model of particulate matter (benzo[a]pyrene adsorbed onto carbon black) induced HO-1 gene expression in a time-dependent manner. Likewise, the addition of benzo[a]pyrene-1,6-quinone, a redox cycling metabolite of benzo[a]pyrene, to RAW cells also induced HO-1. This particle-induced gene expression of HO-1 was found to correlate with a corresponding increase in protein levels. Gene regulation studies were performed to delineate the transcriptional regulation of HO-1 after exposure to model particulate matter. Deletional analysis of the HO-1 gene and mutational analysis of activator protein (AP)-1 regulatory element on both distal enhancers demonstrated the importance of this transcriptional factor in mediating HO-1 gene transcription in response to model particulate matter. These results were supported by gel shift analysis demonstrating increased AP-1 binding activity after exposure to particulate matter. In summary, this study demonstrates that model particulate matter enhanced the expression of HO-1. This inductive process may be mediated by AP-1 activation of the regulatory elements on both the 5'-distal enhancers.