Studies will be directed toward the elucidation of humoral immune mechanisms that may be operative (1) in experimental and human syphilis and (2) following vaccination with gamma-irradiated Treponema pallidum, utilizing the methods of passive protection and neutralization. Studies will also be directed toward the elucidation of $ cell-mediated mechanisms that may be operative utilizing (1) a method for determining the direct cytotoxic effect of lymphocytes upon T. pallidum and (2) anti-rabbit thymocyte globulin as a means of altering immunity and enhancing susceptibility.