Vegetable and animal proteins, injected into rabbits, called forth the production of complement-fixing antibodies in these animals. (a) Gelatin; (b) racemized proteins; (c) proteoses; (d) Vaughan's soluble poison from casein; (e) insoluble residue remaining from the preparation of Vaughan's poison from casein; and (f) animal tissues, did not produce complement fixing antibodies when injected into rabbits. What has been found to be true in the case of other immunity reactions, appears to be true, also, in the case of the complement fixation reaction, namely that the specificity of the complement fixation reaction depends upon the chemical structure of the protein molecule and if the molecule be split or modified by racemization, its specific complement binding power is lost.