Complement (C) originally was detected by its ability to bring about cytolysis of antibody-sensitized bacteria and erythrocytes. Its name is derived from this ability to complement certain reactivities initiated by antibody. At least eleven С proteins, acting sequentially, are involved in mediating the usual antibody-initiated hemolysis. These proteins comprise the classical, or primary, С pathway, which is initiated upon interaction of erythrocyte (E) membrane antigens with receptors on the Fab portion of certain immunoglobulins (Ig), resulting in activation or exposure of a site on the immunoglobulin Fc portion which, in turn, binds with the Clq subcomponent to set the С interactions into motion. This pathway has been divided into recognition (Clq, Clr, Cls), activation (C4, C2, C3), and attack (C5, C6, C7, C8, C9) portions with respect to its function in cytolysis (Muller-Eberhard, 1972), and these interactions have recently been reviewed here (Opferkuch and Segerling, 1976) and elsewhere (Muller-Eberhard, 1975; Mayer, 1973; Ruddy et al., 1972).