Basement membranes are composed of dissimilar protein subunits. The procollagen-like subunit is associated with noncollagenous matrix glycoproteins. The proportion of the latter components varies among basement membranes. The various subunits interact via hydrogen bonds, disulfide bonds and aldehyde-derived cross-links. The extensive degree of cross-linking renders basement membranes highly insoluble. A procollagen-like molecule, extracted from calf anterior lens capsule, exhibits on electron microscopy a filamentous component with a globular portion attached at one end. Treatment of basement membranes with pepsin at low temperature digests the noncollagenous glycoprotein components and allows the collagenous component to come into solution. Purification of the pepsin-solubilized collagen from basement membranes reveals a molecule composed of three identical α-chains. Other unique features include 40–50 residues of hydroxylysine, 128–140 residues of 4-hydroxyproline, 12–15 residues of 3-hydroxyproline, 29 residues of arginine, 35 residues alanine, 2–4 residues of half-cystine, 38 residues of glucosyl-galactosyl-hydroxylysine, 3 residues of mannose, 2 residues of glucosamine, and 0.3 residues of fucose. Immunochemical studies indicate the presence of three distinct antigenic components and support the evidence that one is collagenous and the other two are noncollagenous glycoproteins. One of the latter corresponds to the non-helical extension of procollagen. The other is a large-molecular weight highly cross-linked matrix protein.