Antibody activity is characteristic of the family of structurally related molecules known as the immunoglobulins. These proteins are also known as γ-globulins because of their relative electrophoretic mobility. Many antibodies, however, migrate more rapidly than the electrophoretic γ-globulins (Fig 1). Also, molecules unrelated to antibodies may migrate with the electrophoretic mobility of γ-globulins. For these reasons, the term "immunoglobulin" and the symbol "Ig" or "γ" have been suggested to designate the family of molecules with antibody activity.1 The immunoglobulins include the normal serum components known as IgG (γG, 7S γ2-globulins), IgA (γA, γ1A or β2A-globulins), IgM (γM, 18S γ1-macroglobulins), and IgD (γD) (Fig 1). The immunoglobulins also include myeloma proteins, Bence-Jones protein, and normal urinary components which have no antibody activity but which are structurally related to the antibodies. Investigation of antibodies and of immunoglobulins has advanced rapidly in recent years with (1) separation of major structural subunits of