When Behring's great discovery of antitoxin Opened new paths for the study of immunity it was at once clear that further progress could be attempted in two ways. The first of these, having practical therapeutic results in mind, consists in bending all efforts to the pro duction of various individual curative sera. The other method con sists in seeking a deeper insight into the nature of immunity phe momena, and discovering the general principles underlying the same, for these in turn will aid practical progress.