Introduction. Since it was first recognized that certain substances, such as bacteria or their products or certain other poisonous substances of animal and vegetable origin, on their introduction into the animal body, stimulated the formation of neutralizing bodies, which were termed antibodies, there has been constant investigation of the laws governing such neutralizations. Nevertheless, in spite of the enormous amount of work that has been accomplished in this field, the true nature of neither antigen nor antibody has been definitely established. The present series of papers is concerned with attempts to secure purified solutions of antibody, which would allow direct methods of examination as to their nature and which could be used therapeutically without the introduction of extraneous material, having a dangerous or disagreeable effect. The purification of antibody, as it has been attempted, may be accomplished by the direct and indirect methods.