Four monoclonal antibodies, termed AMH-1, AMH-2, AMH-3, and AMH-4, raised against human lung macrophages in bronchoalveolar lavaged fluid, alveolar spaces, and interstitia of lung tissue are described. The antibodies were produced according to hybridoma technique by immunizing mice with bronchoalveolar lavaged cells. All four monoclonal antibodies reacted with macrophages in bronchoalveolar lavaged fluid and alveolar spaces by immunohistochemical staining and flow cytometric analysis, but they gave different reactivity patterns with the monocyte-macrophage lineage. AMH-1 did not react with peripheral blood monocytes, peritoneal macrophages, or pulmonary interstitial macrophages. Although AMH-2 reacted weakly with blood monocytes and with some of the pulmonary interstitial macrophages, it did not react with peritoneal macrophages. AMH-3 did not show reactivities with either blood monocytes or peritoneal macrophages but was positive for most of the pulmonary interstitial macrophages. AMH-4 was reactive with cells from the monocyte-macrophage lineage. There was a correlation between the reactivity patterns of all four antibodies to macrophages in bronchoalveolar lavaged fluid and the patients' smoking habits. Most significantly, epithelioid cells of lung granulomas obtained from patients with sarcoidosis and hypersensitivity pneumonitis were negative for AMH-1 but were strongly stained by AMH-2, AMH-3, and AMH-4. Differences among the four antibodies in their reactivities with macrophages and granulomas in lungs indicate that lung macrophages contain heterogeneous populations which are in various states of differentiation and maturation and that the epithelioid cells and lung macrophages share the same membrane antigens. Therefore, these antibodies would be useful reagents for investigating the subpopulations and functions of macrophages in lungs and for clarifying the pathogenesis of granulomatous lung diseases.