Helene Sage** and PaulBornstein* abstract: A novel collagen chain, termed aC, has been isolated from human placenta by limited pepsin digestion. The collagen containing the aC chain copurifies with placental AB collagen during selective salt precipitation but is virtually absent from fetal birth membranes, which contain relatively larger amounts of AB. Both native AB and aC-containing collagens are resistant to human skin collagenase under conditions that support cleavage of type I by greater than 90%. The aC chain was separated from aB byphosphocellulose chromatography and subsequently from aA by chromatography on CM-cellulose. Its amino acid composition is distinct from aA and aB although all three chains possess compo--Rl. ecent studies on vertebrate collagen typeshave contributed to an understanding of these proteins as members of a family of closely related but structurally distinct gene products. Evidence for this concept has been provided by detailed structural analyses of collagens isolated froth a variety of in vitro and in vivo systems [for reviews see Miller (1976) and Kefalides (1975)]. Present knowledge supports the existence of three interstitial collagens (types I, II, and III) and at least two basement membrane types (IV and AB) 1 although further studies are necessary to establish the extent to which these categories are distinct. Only type I collagen has been shown to be a heteropolymer comprised of two different chains; however, the subunit structure of the basement membrane collagens has not been resolved (Bentz et al., 1978; Rhodes & Miller, 1978; Burgeson et al., 1976).