A new glycoprotein with an apparent molecular weight of 130,000 was isolated, purified, and partially characterized from the pulmonary secretions which accumulate so massively in the lungs of patients suffering from pulmonary alveolar proteinosis. The amino acid analysis of the glycoprotein showed the presence of relatively high amounts of glycine, glutamic acid, aspartic acid, leucine, and valine, and small amounts of hydroxyproline, but no hydroxylysine. It contains approximately 6% hexose, 3% sialic acid, and 4% glucosamine. The neutral sugars are galactose, mannose, and fucose. This alveolar glycoprotein cross-reacted with an antiserum prepared in rabbits against a larger glycoprotein (250,000 mol wt) isolated from the same source, suggesting that the larger alveolar glycoprotein may be the precursor of the smaller one.