1. Studies of the precipitation reaction of C polysaccharide with C protein, and of C polysaccharide with C antibody are reported. The similarity between these two systems in this respect is demonstrated.
2. The differences between C protein and C antibody are emphasized. The differences between this protein and antibodies in general have been reported previously by others.
3. Electrophoretic studies show that C antibody is in the gamma globulin fraction of serum whereas C protein migrates with the alpha1 globulin fraction of acute phase protein.