Twenty-four rabbits, each immunized intravenously over a 37-week period with a total of 3.71 × 10⁹ γ-irradiated, non-infectious Treponema pallidum, Nichols strain, were found to be completely resistant to intradermal challenge with either 1000 or 100,000 homologous treponemes administered 10 days after the last immunizing injection. Persistence of complete immunity was demonstrable in each of 11 vaccinated rabbits challenged intradermally with 1800 T. pallidum, Nichols strain, 1 year after the last immunizing injection. In contrast, only minimal resistance was observed in 10 of the T. pallidum-immunized rabbits challenged intradermally with 1000 Treponema pertenue, Haiti B strain, 14 days after completion of immunization.

Venereal Disease Research Laboratory, Treponema pallidum immobilization (TPI), and Fluorescent Treponemal Antibody-Absorbed antibodies, each of which is important in the diagnosis and control of syphilis, developed during immunization and persisted in some animals for at least 1 year.

Conclusive evidence that TPI antibody is not associated with the immune response was provided by the finding that 8 of the 11 immune rabbits challenged 1 year after vaccination had no TPI antibody before challenge and failed to develop immobilizing antibody during the 3-month period of observation after challenge.