The chain origins of subunits M1, M2*, and M3 previously described (Butkowski, R. L., Wieslander, J., Wisdom, B.J., Barr, J.F., Noelken, M.E., and Hudson, B.G. (1985) J. Biol. Chem. 260, 3739-3747) of the globular domain of basement membrane collagen were identified, by amino-terminal amino acid sequence analysis, with respect to their relationship to the chains of collagen IV. M1 comprises two polypeptides which correspond to the noncollagenous segments (NC1) of the alpha 1 ad alpha 2 chains of collagen IV. M2*, containing the Goodpasture epitope, and M3 are distinct from these two constituents and from each other but have Gly-X-Y triplets and hydroxyproline at their amino terminus, reflecting the fact that each has a collagen chain origin. These results indicate the presence of two new collagen chains in basement membrane. These new chains appear to be integral components of collagen IV molecules. Alternatively, they could represent new molecular species of basement membrane collagen containing a globular domain, comprising M2* and M3, with physicochemical properties very similar to those of collagen IV.