The heavy polypeptide chains of antibody ( and of gammaG-immnunoglobulin) molecules show discrete bands on disc electrophoresis. The same bands are present for chains from antibodies of the same or diverse specificities. Individual bands are of different intensities for chains from the different rabbits tested even if the antibodies are directed against the same hapten. The bands appear to represent classes of heterogeneous H-chains of the same size having discrete differences in mizobilities with respect to a single charge difference.