Summary: The immunoelectrophoretic method has permitted the definition of a large number of human serum antigens. At least 16 of the more important constituents have been detected with a hyperimmune horse serum. The complete spectrum of serum antigens with any given serum, however, must be obtained by varying the relative concentrations of human serum and the homologous antiserum. Not only can the various constituents be placed in the principal electrophoretic groups characteristic of serum, but their individual mobilities can be determined with remarkable reproducibility. There is only slight variation from one normal serum to another, most frequently in the patterns of the α-2 globulins. Special attention is called to a component of high mobility, well in advance of albumin. This constituent has been named ρ (rho = rapid). Another antigen, component-X, with the same mobility as albumin, is of some interest inasmuch as it is not contained in ethanol preparations of serum albumin. Among the α and β globulins multiplicity is most striking. At least two α-1 globulins, five α-2, four β-1, and at least one β-2 have been detected. The heterogeneity of the albumin is still under investigation. Evidence to date would indicate that the γ globulin, far from being homogeneous, is a group or family of antigenically related, but chemically, physically and biologically different proteins. The immunoelectrophoretic method is demonstrated to be of value to the study of normal antigen mixtures and of pathological and other modifications of these antigens. It is concluded also to be of particular interest for the control of fractionation of such mixtures.