The antibodies which arise naturally following exposure to a disease agent or are provoked experimentally by the introduction of a suitable antigenic material, are said to be an evidence of the immunity which an animal possesses against the antigen. Some workers believe that the antibody is merely an adventitious accompaniment of the immune state of the animal without being essentially related to overcoming or removing the foreign agent and its effects. Others, on the contrary, feel that the antibody is an important functional part of the protective mechanism of the animal, and is instrumental in preventing injury to the animal body. In order to throw some light on this question numerous workers have reported studies in which the time needed by a specifically immunized animal was compared with that required by a normal animal for the disposal of a test injection of antigen. Horse serum has been used as the antigen in many of these investigations.