The time course of antibody synthesis during syphilis was studied in experimentally infected rabbits. A rapid antibody response was seen; the rabbits became positive in both the rapid plasma reagin (RPR) test and Treponema pallidum haemagglutination assay (TPHA) by nine days after infection. Treponemal immobilising antibodies were also seen as early as nine days after infection. Antibody inhibition of treponemal attachment to baby rabbit genital organ (BRGO) cells in culture occurred with immune sera taken 30 days after infection but not earlier. When T pallidum was mixed with immune syphilitic rabbit sera taken at different stages of the infection and used to infect normal rabbits the rabbits became partially resistant to T pallidum only when the treponemes were mixed with sera taken at least 30 days after syphilitic infection. This appearance correlated well with the development of antibodies which blocked attachment of T pallidum to host cells. These antibodies may be involved in the resistance to reinfection which develops in syphilis as the disease progresses.