The effects of granulocyte colony-stimulating factor (G-CSF) on the functional activities of circulating and lung-recruited neutrophils (PMNs) and alveolar macrophages (AMs) were studied in rats to further elucidate the mechanisms underlying G-CSF-enhanced pulmonary host defense. Animals received G-CSF or vehicle twice a day for 2 days, followed by an intratracheal challenge with endotoxin or saline. G-CSF up-regulated CD11b/c expression and mean channel fluorescence intensity of phagocytosis in circulating PMNs. G-CSF also enhanced phagocytic activities, reflected by both the percentage of phagocytosis and mean channel fluorescence intensity in lung-recruited PMNs and AMs in intratracheal endotoxin-challenged rats. The endotoxin-induced increase in pulmonary production of tumor necrosis factor-alpha and cytokine-induced neutrophil chemoattractant was not affected by G-CSF pretreatment. These data demonstrate that G-CSF-enhanced pulmonary recruitment of PMNs is primarily based on the effects of G-CSF on the PMNs themselves, rather than the generation of certain chemotactic stimuli, i.e., cytokine-induced neutrophil chemoattractant and tumor necrosis factor-alpha. The enhanced phagocytic activities of lung-recruited PMNs and AMs also augment pulmonary host defenses in G-CSF-pretreated animals.