The heavy chain (-y3) of the IgG3 subclass of human immunoglobulins has a molecular weight of 60,000, instead of the 50,000 value reported for -yl, y2, and The heavy chains of IgG (y) consist of four homology regions or "domains," each of which is made up of about 110 aminoacid residues, and is characterized by a single, highly conserved intrachain disulfide bridge (1). The amino-terminal domain, also known as the variable region (VH), participates in the formation of the antibody-combining site. The constant region, containing three domains (CH1, CH2, and CH3), is identical for molecules belonging to a given subclass and has approximately 90% homology from one y chain subclass to another (2, 3). Between the CH1 and CH2 domains, in the center of the 7 chain, is a region of unique sequence known as the "hinge" or interdomain, which has no homology with any known H or L chain domain. In the different subclasses of y chains, it contains a large number of proline residues, and a varying number of cysteine residues involved in the disulfide bridges linking the two heavy chains (4,5).Previous studies of a 73 myeloma protein (Kup) demonstrated the presence of five cysteine residues in a 33-residue Abbreviations: Nomenclature of immunoglobulins, their chains, and fragments follows the recommendations of the World Health Organization [WHO Bull. (1965) 33, 721; (1966) 35, 953; 38, 151; (1969)