The isolation and characterization of type III collagen from adult human lung parenchyma are described. The identity of this molecule as type III collagen was established on the basis of demonstration of intramolecular disulfide cross-links in the helical portion of the molecule, amino acid analysis characteristic for type III collagen and composition and size of isolated cyanogen bromide peptides .alpha.1(III)-CB3, .alpha.1(III)-CB5 and .alpha.1(III)-CB8. The MW of lung .alpha.1(III) was determined as 93,000 by agarose chromatography but its electrophoretic mobility on sodium dodecyl sulfate-polyacrylamide gels was slower than that of type I .alpha. chains which also have a MW of 93,000.