Lung macrophages obtained by segmental lavage from three patients with pulmonary alveolar proteinosis were studied in vitro. The macrophages exhibited morphologic abnormalities including excessive lipid accumulation and giant secondary lysosome formation. These cells survived poorly in tissue culture, showed impaired chemotactic activity, and had decreased adhesiveness to glass. They phagocytized normally but had substantially decreased capacity to kill ingested Candida pseudotropicalis. Evidence was obtained that the macrophage defect was acquired and probably related to ingestion of the proteinaceous alveolar fluid. Peripheral blood monocyte function was normal in one patient and morphologic abnormalities were produced in normal monocyte-derived macrophages cultured with proteinaceous lavage material. These studies suggest that the lung macrophage in alveolar proteinosis is a defective cell as a consequence of an abnormal pulmonary environment.