Summary: Four γ-globulin fractions of human serum which differ slightly in average electrophoretic mobility at pH 8.6 have been investigated by physical and immunochemical methods. Each preparation consists of several antigens, and the quantitative immunological behavior of each fraction is determined by the relative proportions of the various components present. The difficulties involved in using immunological methods for assay of γ-globulin in body fluids are discussed. The various fractions differ from one another most strikingly in the region of antigen excess.