Pulmonary alveolar proteinosis is characterized by the accumulation of granular proteinaceous material within the alveoli of the lung. It is well established that patients with pulmonary alveolar proteinosis have a high incidence of complicating pulmonary infections, which suggests that the function of the alveolar macrophages is abnormal. To investigate the function of these cells, they were obtained from two patients by pulmonary lavage with physiologic saline solution and were incubated with Staphylococcus aureus in vitro. The decline in viable organisms from the culture was measured and compared with results obtained with normal alveolar macrophages. On the basis of decreased phagocytosis, results indicate that pulmonary alveolar macrophages from these patients had defective antibacterial function.