1. Detailed physical, chemical, and immunologic studies of a protein closely related to the Fc fragment and heavy chain of G immunoglobulin (IgG), and elaborated by a subject with a lymphoproliferative disorder are presented. 2. The protein, which has a molecular weight of 51,000, was cleaved into two half molecules by reduction and alkylation. 3. The protein has few if any of the antigenic determinants of the antigen-binding (Fab) papain fragment of IgG, and has a striking similarity in its antigenic properties to the Fc fragment. 4. Fingerprint patterns resemble those of the crystallizable (Fc) fragment, and lack several peptides found in the heavy chain. 5. These findings suggest that the Fc fragment may be a real structural unit of IgG, and raise the possibility of the existence of three different types of polypeptide chains in G immunoglobulin.