Two populations of IgG molecules, designated F-IgG3(χ) and S-IgG3(χ), were isolated from a patient with multiple myeloma. The constant regions of the two molecules appear to be identical, since the heavy chains of both belong to the γ3 subclass and both molecules are positive for G3m(5), G3m(10), and G3m(ll) but negative for G3m(21), Km(l) and Km(2) allotypes. In contrast, the two molecules have different variable regions, since (a) they do not share idiotypic determinant(s); (b) their light chains have different NH2 terminal amino acid sequences, and (c) both their heavy and light chains have different peptide maps and amino acid compositions. SDS-polyacrylamide gel electrophoresis showed that the F-IgG3(χ) molecules have two kinds of light chains. One, designated χn, has a normal molecular weight (approximately 23,000); the other, designated χh, has a molecular weight of approximately 2