THE specific (γ) polypeptide chain of immunoglobulin G (IgG, ref. 1) molecules carries several genetic markers (Gm antigens)2,3 which can be detected by serological techniques. Each of the markers is associated with one of three subclasses of γ chain, called γ1, γ2 or γ3 chains4. (γ1, γ2, γ3 and γ4 are approved designations for the heavy chains of the immunoglobulins IgG 1, IgG 2, IgG 3 an IgG 4, respectively5.) Most Gm antigens have been localized in the Fc region of the IgG molecule; the exceptions are the Gm(z) and (f) markers which are located in the Fd region. At present no genetic marker is known for γ4 chains.