The problem of immunity in Syphilis is a very complex problem which is tightly bound to healing. But for lack of being able to grow T. pallidum and as we are without any absolute criterion of bacteriological sterilization, the only method we have to be able to consider this study is to have recourse to experimentation upon animals. Two theories had been proposed to give an explanation about this phenomenon occurring during a late treated syphilis: on one side, for Kolle, Evers and Neisser, immunity in syphilis would be due to a premunition given by the only persistence in the organism of T.p. having still kept all their virulence. On the other hand, as far as Chesney and Kemp are concerned it would be a true immunity, obtained before the application of any therapeutic treatment, but which is not due to persistence of T.p. in tissues. However, recent works have proved, and this has been confirmed by several searchers, that a penicillin therapy, even at a high dose, given six months after the infection outset, is no longer able of destroying all T.p. Immunity in syphilis would then be due to a modification of tissular receptivity kept by the persistence of T.p. vegetating in the organism in a commensal state. As far as the nature of this immunity is concerned, all the present research would lead to prove that this refractory state would not be due to tumoral properties, but more likely to tissular properties and probably to cell mediated phenomena. Similar research carried out in man has created an opportunity to demonstrate these facts similar to those observed in experimentations upon animals. They particularly prove that a persistent serology over a long period is not due to a "cicatrice sérologique" (Serological marker) but to the keeping of T.p. in a quiescent state. However, it is a more or less relative immunity which is concerned, depending upon a very large number of factors that are far from being elucidated.