GENETIC polymorphism of the structural genes encoding the class-specific (heavy) polypeptide chains of the immunoglobulin (Ig) molecules provides a useful set of markers for elucidating the arrangement and expression of these genes. On the basis of various antigenic, physiochemical and biological properties, the immunoglobulins of the mouse have been divided into eight distinct classes, IgM, IgD, IgA, IgG1, IgG2a, IgG2b, IgG3 and IgE, with a specific structural gene determining the heavy chain (H) for each class. Previous studies have documented the existence of a genetic polymorphism for five of these H-chain genes1–3, with all the genes being closely linked constituting a heavy chain chromosome region. The most recent of these loci to be demonstrated (Ig-5, determining the δ chain)3 was detected by the reaction of alloantisera prepared against spleen cells from mice differing in both H–2 and allotype. These sera reacted with B lymphocytes of the appropriate allotype congenic strains, as assessed by either immunofluorescence, or cell surface iodination and polyacrylamide gel electrophoresis (PAGE). As most B lymphocytes express cell surface IgM as well as IgD (refs 4–6), we have further pursued these various approaches to identify Ig allotypes and have found a polymorphism of the heavy chain (μ) of murine IgM, a molecule found in pentameric form (19S) in serum, and in monomeric (8S) form on the surface of lymphocytes. This defines a locus, Ig-6, which encodes the μ chain of most mouse IgM immunoglobulin.