The knowledge of the effect of subcutaneous and, inadvertently, intradermal injection of diphtheria toxin on the cutaneous tissue, is as old as the knowledge of diphtheria antitoxin. It was necessarily one of the first observations made during the process of immunization and of testing out the strength of diphtheria antitoxin. A great deal of the knowledge accumulated at that time has a distinct bearing on the interpretation of the phenomena now seen in the cutaneous reaction known as the Schick test.
Soon after the appearance of the reports concerning the clinical value of the intradermal diphtheria toxin test, I became interested in studying the character of the reaction and its behavior under the influence of repeated injections. My interest was held because of the occurrence of happenings analogous to those encountered with repeated injections of horse serum and cowpox vaccine, which had to be explained on entirely different grounds, and