Particulate air pollution (PM(10)) stimulates alveolar macrophages (AMs) to release immature granulocytes from the bone marrow (BM) into the circulation. This study was designed to determine the effect of PM(10) (ambient EHC-93 or inert carbon [CC]) instillation exposure on the monocyte release from the BM and the role of AM in this response. Monocyte precursors were labeled in the BM of rabbits in vivo by an intravenous injection of 5'-bromo-2'-deoxyuridine, and the effects of PM(10) were determined by instillation either particles or supernatants of AM exposed to particles into the lungs. Instillation of EHC-93 (500 microg/ml) or supernatants from AM incubated with EHC-93 (100 microg/ml) increased circulating band cell counts (p < 0.05) and shortened the transit time of monocytes through the BM (35.5 +/- 2.2 to 25.0 +/- 1.5 hours or 36.2 +/- 2.6 to 25.7 +/- 1.8 hours, p < 0.05) compared with the control subject. CC (1%) instillation also shortened the monocyte BM transit time to 28.4 +/- 1.9 hours (p < 0.05), but supernatants of AM incubated with CC did not. We conclude that exposure to atmospheric PM(10) stimulates the production of mediators by AM, and these cytokines accelerate the monocyte release from the BM.