Intradermal infection of rabbits with Treponema pallidum imitiates rapid and active cellular response at the site of injection. During the first 2 weeks following infection, there is a marked increase in the numbers of organisms at the site of infection. Systemic dissemination of treponemes occurs during the early stage of infection, presumably before the immune response is fully mobilized. The mononuclear infiltration, which is apparent at the lesion site one week postinfection, becomes more pronounced at 2 weeks. The infiltrating cells are predominantly T lymphocytes and macrophages. By 4 weeks postinfection, most of the organisms have been cleared from the primary site; however, low numbers of treponemes survive locally and in distant tissues. Thus, whereas infection with T. pallidum appears to activate immune mechanisms which are capable of clearing most of the organisms from the primary lesion, some organisms are able to evade these mechanisms and persist in vivo.