1. Two horses were injected subcutaneously with alum-precipitated rabbit serum albumin.
2. The resulting antibody resembled diphtheria antitoxin and anti-egg albumin in the horse in giving a sharp zone of flocculation with antigen, in being water-soluble, in reactivity toward an anti-antibody rabbit serum, and in its electrophoretic properties.
3. The effect of continued immunization, and of variation in volume and temperature on the reactivity of the antibody are discussed.
4. Intravenous injection of the same antigen into horses did not give rise to detectable amounts of antibody of the same type.