1. The intravenous injection of two horses with alum-precipitated rabbit serum globulin resulted in the production of antibody which gave a typical precipitin reaction without a prezone in the region of antibody excess.
2. The chemical, physical, and serological properties of this antibody are comparable to those of the more familiar anticarbohydrate antibodies.
3. The subcutaneous injection of horses with the globulin antigen gave rise to low grade "univalent" antibody which did not precipitate with soluble antigen.
4. The low grade antibody could be removed from solution by attachment to preformed specific precipitates, or by coprecipitation in the presence of "multivalent" precipitating antibody.
5. It is concluded that the familiar antitoxin type of antibody is not the only form of antiprotein response in horses but that precipitating and low grade non-precipitating antibodies may also be formed.
6. The nature of the antigen and the route of injection are demonstrated to be important factors in determining the characteristics of the antibody formed.