Rabbits were immunized over a 32-week period with a total of 450 micrograms of purified Treponema pallidum endoflagella. As measured by enzyme-linked immunosorbent assay, sera from immunized rabbits had antiendoflagellar antibody titers that were fivefold greater than titers of sera from infected immune rabbits and patients with secondary disease. Sera from all immunized animals possessed complement-dependent treponemicidal activity as measured by in vitro immobilization. Immunized animals challenged with virulent T. pallidum were not protected from symptomatic infection but showed an altered course of lesion development.