The heavy chain (gamma-3) of the IgG3 subclass of human immunoglobulins has a molecular weight of 60,000, instead of the 50,000 value reported for gamma-1, gamma-2, and gamma-4 heavy chains. By use of protein Omm, a gamma-3 heavy chain disease protein, it was possible to isolate and analyze the extra fragment. Protein Omm had a molecular weight of 40,000, glycine as its sole NH-2-terminal, and contained only the hingee region and the C-H-2 and C-H-3 domains. CNBr cleavage at Met 252 (gamma-1 numbering) yielded the hinge fraction (Fh fragment). On the basis of the molecular weight of Fh (11,000), its amino-acid composition, its partial sequence, and its unexpectedly low number of tryptic peptides, it is postulated that the extra fragment in gamma-3 heavy chains represents a series of similar or identical duplications of sections of the previously reported gamma-3 hinge region. In addition, there are striking homologies with the hinge region of alpha-1 and alpha-2 heavy chains, one of which also has duplications. The relationship of these hinge structures in different immunoglobulins supports the concept that this region is coded by a unique, small piece of DNA, which has evolved in parallel manner with the immunogolbulin genes by partial duplications and/or crossingover.