The α1(I), α2, and α1(III) collagen chains are retarded in urea-SDS gels relative to their migration in SDS gels without urea (Hayashi and Nagai, 1979). This effect of urea provides the rationale for the two-dimensional separation of collagenous from non-collagenous polypeptides by two-dimensional electrophoresis consisting of urea-SDS in one dimension and SDS alone in the other dimension. In this system, non-collagenous polypeptides align themselves diagonally across the separation plane while each class of α chain with its homologous crosslinked components, forms a separate and distinct alignment at an angle to the central diagonal. © 1981, Gustav Fischer Verlag, Stuttgart/New York. All rights reserved.