Introduction In the preceding article (1), data were presented dealing with some aspects of the union between specific antigen and the skin of protein-immunized rabbits. Horse serum was employed as the immunizing protein and horse serum antitoxin as the test antigen. It was believed that in testing the skin of the horse serum-immunized rabbits with antitoxin instead of horse serum, a more quantitative picture could be obtained of the skin response, since antitoxin can be measured quantitatively by its toxinneutralizing action. In practice, a lethal amount of diphtheria toxin and a neutralizing quantity of antitoxin were injected some distance apart into the skin of normal and of horse serumimmunized rabbits. The normal animals survived and the immunized animals succumbed. In the former animals, both the toxin and antitoxin diffused from the area of injection and the toxin was evidently neutralized in vivo.