A glycoprotein of Mr 62 000 was isolated from lung lavage material of patients with alveolar proteinosis. The glycoprotein was found to contain (per molecule) 72 residues of glycine, 5 residues of hydroxyproline, 3 molecules of sialic acid, 4.9 molecules of mannose, 4.0 molecules of galactose, 0.9 molecule of fucose and 7.0 molecules of N-acetylglucosamine. Limited pepsin digestion of the glycoprotein resulted in six peptides, three of which contained hydroxyproline and nearly 30% glycine, and two of which contained all the carbohydrate present in the glycoprotein of Mr 62 000. The three peptides containing hydroxyproline and with high content of glycine contained a repeating -Gly-X-Y-sequence in the peptide chain. Partial amino acid-sequence analyses on the peptides derived from the digestion of the alveolar glycoprotein with various proteolytic enzymes indicate that this glycoprotein is characterized by the presence of alternating collagenous and non-collagenous regions in the same polypeptide chain.