Human immunoconglutinin was titrated by its reaction with sensitized sheep erythrocytes alexinated with horse complement. IC was shown to be a euglobulin and to be sensitive to the action of 2-mercaptoethanol. On Sephadex G200 chromatography and density gradient ultracentrifugation, its behavior was that of a macroglobulin. IC activity was absorbed by an antiserum to human IgM globulins. In contrast to bovine conglutinin, IC did not require calcium ions for its action, and was not inhibited by N-acetyl-D-glucosamine. Sequential complement fixation at least as far as AgAbC′1423 is required for IC to react with alexinated complexes. Reactivity with autologous complement was demonstrated.