A carbohydrate-rich, water-soluble glycoprotein has been isolated in pure form from delipidated lung lavage fluid from a patient with pulmonary alveolar proteinosis, in a three-step procedure involving ion-exchange and gel filtration chromatography. The molecular weight of the glycoprotein was determined to be 45 900 by sedimentation equilibrium analysis in the analytical ultracentrifuge and 45 000 by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, indicating a single polypeptide chain. Nearly half of the mass of the glycoprotein is comprised of carbohydrate that is contributed by 24 residues sialic acid, 23 residues N-acetylglucosamine, 6 residues N-acetylgalactosamine, 19 residues galactose, 4 residues mannose, 1 residue fucose and 1 residue glucose per mol. Unlike a number of collagen-related glycoproteins that have been isolated by others from insoluble lung contents in pulmonary proteinosis, the water-soluble glycoprotein described in the present report does not contain hydroxyproline or hydroxylysine and contains less than 10% of its amino acid residues as glycine. Using rabbit antibodies directed against our purest preparation of material and an immunoperoxidase staining procedure, the 45 000 molecular weight glycoprotein was localized to the thin film of fluid lining the surfaces of alveoli in normal human lungs.