INTRODUCTION The vast amount of literature dealing with the problem of immunity in syphilis may be grouped into a series of extensive clinical observations, on the one hand, and on the other, the report of a considerable amount of experimental work which received its first impulse from the pioneer studies on syphilis transmission in chimpanzees by Metchnikoff and Roux1two years prior to Schaudinn's discovery ofSpirochaeta pallida. The conclusions, which were drawn from both the clinical and experimental data that had accumulated during a period of years, were, in the first instance, the result of observations not subject to control, and, with the causative agent yet unknown, could not be interpreted scientifically; in the second instance, the experimental findings, without exception, were based on what is now known to be an erroneous conception of the nature and behavior of the syphilitic virus within the animal body. Reinoculation in