The morphologic identity of Spirochaeta pallida and Spirochaeta cuniculi and the resemblance between the processes caused by these micro-organisms, respectively, in the animal body cast a shadow of doubt, at the very beginning, on the value of the results obtained from the study of experimental syphilis in the rabbit. More than that, they have led Arzt and Kerl, Klarendon and Simon and others to the belief that the existence of a spontaneous spirochetosis in the rabbit makes this animal altogether useless for the study of experimental syphilis. However, the majority of investigators (Jacobstahl, Kolle, Mulzer, Uhlenhuth and others), basing their results on comparative studies of spontaneous spirochetosis and of experimental syphilis, have expressed the opinion that in spite of the morphologic resemblances of both strains, the contrast of the diseases that they cause respectively is rather sharp, and the margin of error in their discrimination is practically nil.