To judge the pathogenicity of Treponema palllidum in mice, we inoculated two groups of animals. 1) One with a normal immune system : adult Swiss mice. 2) The other with an immune system presenting a defect in T cells : athymic Nude mice. No lesions appeared at the points of inoculation and the survival of these animals was not modified compared with control animals. We have also carried out a study of antitreponema antibodies in adult and new-born Swiss mice, using, for the adults, various concentrations of treponema pallidum and various routes of inoculation. In adult mice, the search for antibodies by fluorescence, showed, firstly, that the intradermal route permits a more rapid humoral response than the intraperitoneal route, with higher antibody titers. Furthermore, an inoculum of 8,6 X 10(6) per mice, gives earlier results and higher titers than those obtained with an inoculum twice lower. The earliness of the positive Nelson test is independent of the route of inoculation but related to the injected dose. On the other hand, the Kline reaction remained negative for all groups of mice throughout the whole period of our study. In new-born mice, 76% only became positive on fluorescence and Nelson test.