Rabbits infected intradermally with 10(3) Treponema pallidum (Melbourne 1) cells were examined for their susceptibility to reinfection with 10(2) T. pallidum cells (homologous or heterologous strains) at various intervals after the initial infection. At 2.5 weeks after infection, the rabbits were extremely sensitive to reinfection and developed syphilitic lesions significantly faster (i.e., shorter latent periods) than control rabbits that had not received the initial infection. This phenomenon may represent a state of immunosuppression or hypersensitivity in the infected rabbits. Whatever its etiology (at present unknown), it was a transient state since at 5 weeks after infection the rabbits were no longer different from control rabbits in their susceptibility to reinfection. They showed neither immunity (i.e., longer latent periods) nor immunosuppression or hypersensitivity (shorter latent periods) upon reinfection. At 6.5 weeks after infection, two of the three experimental rabbits were fully immune (no lesions upon reinfection), whereas the other rabbit exhibited immunosuppression or hypersensitivity upon reinfection. At 7.5 and 10 weeks after infection, all of the experimental rabbits were immune to reinfection. We conclude that syphilitic rabbits show a biphasic response to reinfection, consisting of an early phase of enhanced sensitivity to T. pallidum and a later phase of immunity to T. pallidum.