Immunoelectrophoretic analysis by horse or rabbit antibodies of human γ-globulin suggests antigenic homogeneity, or at least a high degree of antigenic correspondence, over a broad range of electrophoretic mobilities. Quantitative electrophoretic homogeneity and qualitative immunological purity are not necessarily of the same significance. A fraction such as γ2-W, which contains several non-γ impurities, may show a quantitative homogeneity of the same order as that of II-Sq, which is comprised uniquely of γ-globulin with asymmetric electrophoresis dispersion. Subfractionation of a preparation of II-Sq into water soluble and water insoluble material at pH 7.2, and the separation of serum euglobulin from fresh serum gave evidence for the existence of complexes among human serum antigens, or for their formation by the fractionation procedures. The γ-globulins of different solubility appeared to have the same antigenic specificity. The significance of serum complexes is discussed in relation to the nature and function of γ-globulin.