When Paul Ehrlich1 in his. early experiments with dyestuff and tissue staining succeeded in simplifying the identification of the blood cells, the basis of hematology was established. The application of these principles to the microchemical reaction of tissue to dyestuff resulted in the introduction of the theory of selective affinity between chemical substances and protoplasm. Moreover, the observations from all those experiments led Ehrlich to outline his side-chain theory. In spite of the adverse criticisms of this theory, it proved to be a valuable link in the development of the science of immunity and serum reactions. After the discovery of immune bodies in syphilized blood, Ehrlich assumed that special antitoxins would not be effective in treating protozoan diseases. This conclusion initiated a series of trial and error experiments to find a therapia sterilisans against syphilis. As arsanilic acid was useful in the treatment of spirillar infections, compounds of this