Immunoconglutinin is an antibody against fixed complement which appears in the serum of an animal after a strong antigenic stimulation. Using alexinated sheep red blood cells as antigen, various reactions between rabbit immunoconglutinin and complement were examined. It was shown that immunoconglutinin acted as an antibody to bring about the binding of horse, rabbit, and guinea pig complement. In the presence of hemolytic complement, immunoconglutinin had hemolysin activity.Immunoconglutinin was specifically absorbed from immune serum by using the antibody and complement present in the serum, thus demonstrating that immunoconglutinin can react with autologous complement.Rabbit sera containing heterostimulated immunoconglutinin showed an increased and prolonged bactericidal effect. It is suggested that the function of immunoconglutinin is to enhance the normal activities of complement.