Summary The ability of complement from various animal species to precipitate soluble rabbit antigen-antibody complexes in the region of antigen excess has been demonstrated. Guinea pig and rabbit complements precipitated the soluble complexes better than human or pig complements, and guinea pig complement precipitated the complexes better than rabbit complement. The failure of pig and human complements to precipitate soluble complexes to the same extent as guinea pig and rabbit complements may be partially caused by several factors: (1) an inhibition resulting from cross reactions between the antibodies in the complexes and the human and porcine proteins, (2) the presence of an inhibitory factor which was found to be inactivated upon aging the serum, and (3), the physical and chemical characteristics of complement molecules themselves. Two possible hypotheses were set forth to describe the mechanism involved in the precipitation of soluble complexes by complement. These hypotheses were: (1) complement fixed to the complexes to such an extent that the increase in weight and/or neutralization of charges caused the complexes to precipitate, and (2), the complement was multivalent to antibody and/or antigen and, therefore, was able to link the complexes together to form precipitating aggregates. The ability of guinea pig and human complement to cause precipitation of “univalent” (co-precipitating) antigen-antibody complexes was demonstrated.