Summary: Data obtained from kinetic studies of C′ fixation with a polysaccharide immune system are presented to show that the C′-fixing potency of an immune serum may be related to the aggregating properties of the antibody. The evidence at hand points to the fact that antibody which flocculates readily with homologous antigen not only fixes C′ efficiently but is more readily subject to a steric hindrance effect in the fixation of C′ by preformed aggregates. In addition, experiments are described which indicate that the variations in the specific C′-fixing potency observed in different sera may also exist among different portions of the antibody in a single immune serum.