Recent studies have shown that an increased concentration of ambient particulate matter (PM10) is related to decreased pulmonary function and respiratory and cardiovascular mortality. The mechanisms responsible for this excess mortality are unknown and the relationship between the level of PM10 and the circulating leukocyte counts has not been previously investigated. We postulated that the deposition of PM10 in the peripheral lung stimulates alveolar macrophages (AM), which results in polymorphonuclear leukocyte (PMN) release from bone marrow (BM). To test this hypothesis, either colloidal carbon (CC) (n = 3) or saline (n = 4) was instilled into the lungs of rabbits and PMN release from BM was evaluated by using 5'-bromo-2'-deoxyuridine (BrdU). CC instillation in the lung shortened the transit time of PMN through the BM to 71.0 +/- 6.9 h compared with the saline controls (85.5 +/- 2.8 h, p < 0.01). The role of AM in this response was further investigated by incubating isolated AM in tissue culture medium either with or without the presence of CC, and measuring the effect of the supernatants on the release of PMN from the BM. The supernatant of AM incubated with CC shortened the PMN transit time through the BM to 74.9 +/- 3.7 h (p < 0.05) compared with the supernatant from the unstimulated AM (98.6 +/- 1.9 h) and medium alone (94.3 +/- 3.7 h). We conclude that the phagocytosis of CC by AM releases mediators (cytokines) that stimulate the BM to release PMN. We speculate that these newly released PMN may play an important role in the decline in lung function and high mortality seen in populations exposed to high concentrations of atmospheric PM10.