1. Complement is not fixed by immune aggregates resulting from the interaction of pneumococcus capsular polysaccharide and type-specific immune horse serum, although under proper conditions the substitution of immune rabbit serum gives positive results.
2. The negative results with immune horse serum are due to some poorly understood property of the specific antibodies rather than to some heterologous inhibitor present in the serum.
3. It has been shown that with immune rabbit serum-polysaccharide combinations, complement fixation is an adsorptive phenomenon conditioned upon the surface exposure of the immune aggregates.
4. A close parallelism to the selective adsorption of phosphatides by these immune aggregates has been pointed out.
5. In those instances in which complement is fixed this phenomenon must be regarded as tertiary and conditioned by (a) union of antigen and antibody, and (b) particulation.
6. The general significance of complement fixation as applied to bacterial polysaccharides has been discussed.