The nature of the cellular infiltration and the distribution of Treponema pallidum during rabbit testicular infection were examined by immunofluorescence and light microscopy. Low numbers of treponemes are demonstrable in the perivascular regions on day 3 post-infection. On days 6, 10, and 13, large numbers of organisms are found in the interstitial spaces. The treponemes do not appear to invade the walls or lumina of the seminiferous tubules, although tubular atrophy is obvious. On days 17, 24 and 31, treponemes are no longer identifiable by immunofluorescence in infected testicles. The cellular infiltration, which is apparent on day 6, reaches its peak on day 13, corresponding to the amount of swelling observed grossly. The infiltrate is primarily lymphocytic, but macrophages are also observed during the peak cellular response. These cells are located, as are the treponemes, in the interstitial spaces. The lymphocytes are demonstrated by specific immunofluorescence to be predominantly T cells. The peak T cell infiltration in the infected testicle is followed rapidly by the disappearance of the organisms from that organ. Thus, it is postulated that infiltration by specifically sensitized T cells results in the clearance of large numbers of T. pallidum from infected tissues.