Pulmonary alveolar proteinosis (PAP) is characterized by accumulation of large quantities of lipoproteinaceous materials in alveoli. Surfactant protein A (SP-A) is the predominant phospholipid-associated glycoprotein in pulmonary surfactant and is specific to the lung. The contents of SP-A in bronchoalveolar lavage (BAL) fluids of patients with PAP were measured with an enzyme-linked immunosorbent assay using two monoclonal antibodies to human SP-A to evaluate its usefulness for diagnosis. Concentration of SP-A in BAL fluid in PAP was significantly increased in comparison with that of normal volunteers. The ratio of SP-A to protein in BAL fluid of PAP was at almost the same level as in normal subjects, while the ratio of SP-A to phospholipid in PAP was significantly higher. These results indicate that measurement of BAL fluid SP-A is of clinical value for diagnosis of PAP and should be used as a biochemical diagnostic tool in the clinical laboratory.