We have determined the amino acid sequence of the lambda light chain of human IgD WAH. Together with the sequence of the delta heavy chain already reported, this establishes the complete covalent structure of a human immunoglobulin D. The sequence determination was greatly aided by our ability to use high-pressure liquid chromatography to purify large peptides, including one large fragment extending from Ser-81 through the carboxyl terminus. The IgD molecule is a four-chain monomer of Mr approximately equal to 176,000; it consists of two lambda chains (each of 214 residues, Mr = 22,893) and two delta chains (each of 512 residues, Mr approximately equal to 65,000, including carbohydrate), and, unlike murine IgD, it contains two C delta 2 domains. A computer-assisted search using the J (joining) segment of the WAH lambda chain as a test piece showed a close evolutionary relationship of human and mouse J lambda regions and suggested that germ-line J lambda genes in the two species are very similar if not identical. DNA segments encoding J lambda, J kappa, and JH appear to have had a common evolutionary origin, and, surprisingly, JH seems closer to J lambda than does J kappa.