Background : It has long been suggested that neutrophils and their products are implicated as the central mediators of the acute lung injuries. Contrary to the dominant role of neutrophils in ARDS, many cases of ARDS has occurred in the setting of severe neutropenia without pulmonary neutrophil infiltration. Therefore it is certain that effector cell(s) other than neutrophil play an important role in the pathogenesis of ARDS. This experiment was performed to define the mechanism of ARDS in the setting of neutropenia, 1) by comparing the severity of endotoxin-induced lung injury, 2) by measurement of hydrogen peroxide production and cytokine concentration in the bronchoalveolar lavage cells and fluids obtained from different rats with and without cyclophosphamide-pretreatment. Method : The male Sprague-Dawleys were divided into the normal control (NC)-, endotoxin (ETX)-, and cyclophosphamide (CPA)-group in which neutropenia was induced by injecting cyclophosphamide intraperitoneally. Acute lung injury was evoked by injecting lipopolysaccharide (LPS) into a tail vein. The bronchoalveolar lavage (BAL) was performed at 3 and 6 hour after administration of LPS to measure the change of cell counts and concentrations of protein and cytokines, tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6). Hydrogen peroxide (HPO) production from BAL cells was measured at 6 hour after LPS administration by phenol red microassay with and without zymosan stimulation. Results : The results were as follows. A change of leukocyte counts in the peripheral blood after treatment with CPA : More than 95% of total leukocytes and neutrophils were reduced after CPA administration, resulting in severe neutropenia. A change of BAL cells : In the ETX-group, the number of total cells (p < 0.01) and of macrophage and neutrophil (p < 0.05) were increased at 3 and 6 hour after LPS administration compared to those of NC-group. In the CPA-group, the number of total leukocyte and macrophage were not changed after LPS administration, but neutrophil counts were significantly reduced and it took part in less than 0.1% of total BAL cells (p < 0.01 vs NC-group). BAL cells in this group were almost all macrophages (99.7%). A change of protein concentration in the BALF : In the ETX-group, protein concentration was increased at 3 hour and was more increased at 6 hour after LPS administration (p < 0.05 and < 0.01 vs NC-group, respectively). In the CPA-group, it was also significantly elevated at 3 hour after LPS administration (p < 0.05 vs NC-group), but the value was statistically not different from that of ETX-group. The value measured at 6 hour after LPS administration in the CPA-group became lower than that of ETX-group (p < 0.05), but showed still a higher value compared to that of NC-group (p < 0.05). A change of cytokine concentration in the BALF : TNF -alpha and IL-6 were elevated in the ETX - and CPA-group compared to those of NC-group at both time intervals. There was no statistical difference in the values of both cytokines between the ETX- and CPA-groups. Measurement of hydrogen peroxide production from BAL cells : There was no intergroup difference of HPO production from resting cells. HPO production after incubation with opsonized zymosan was significantly elevated in all groups. The percent increment of HPO production was highest in the ETX-group (89.0%, p < 0.0008 vs NC-group), and was 42.85 in the CPA-group (p = 0.003 vs NC-group ). Conclusion : Acute lung injury in the setting of neutropenia might be caused by functional activation of resident alveolar macrophages.