Abstract Six γM-globulins of antigenic Type K and one of Type L from patients with Waldenstrom's macroglobulinemia were purified, and their heavy (µ) and light κ or λ) chains were separated by reduction-alkylation and gel filtration. The purified γM-globulins and their isolated polypeptide chains were characterized by starch gel electrophoresis, immunological study, amino acid and NH2-terminal group analysis, and peptide maps. The γM-globulins were generally similar in amino acid composition but differed markedly from their constituent heavy and light chains and also from γG- and γA-globulins. The Type K (κ) light chains had NH2-terminal aspartic acid (five cases) or glutamic acid (one case), but no NH2-terminal group was detected in the Type L (λ) light chain and some of the heavy chains. For each γM-globulin the heavy and light chains were complementary in amino acid composition and peptide maps with respect to the whole protein, but differed greatly from each other. All of the heavy chains had about two-thirds of their soluble tryptic peptides in common, but differed individually with respect to the others. The κ light chains shared some peptides, all of which were identifiable with known sequence areas in Type K Bence-Jones proteins, but, like the latter proteins, they differed in other peptides representing the variable NH2-terminal half of the chain. The light chain of one γM-globulin and the Bence-Jones protein from the same patient were similar in many properties. The results support a multichain structural model of γM-globulins composed of two light chains of about 23,000 mol wt and two heavy chains. Each kind of chain appears to have a structural region subject to individual variation.