Highly purified C'1 esterase of human serum is capable of inactivating isolated fourth component of human complement (beta(1E)-globulin). Inactivation is accompanied by changes in electrophoretic and ultracentrifugal properties of beta(1E)-globulin. If non-sensitized sheep erythrocytes are present during the action of C'1 esterase on beta(1E)-globulin, a complex is formed consisting of cells and cytolytically active fourth component (EC'4). Thus, inactivation of beta(1E)-globulin by C'1 esterase appears to be preceded by a state of activation enabling beta(1E)-molecules to combine with cell membrane receptors. Acceptor groups appear to be present also in 7S gamma-globulin and in beta(1E)-globulin itself, since C'1 esterase can induce the formation of beta-beta and of beta(1E)-7S gamma-globulin complexes.