Three female rabbits, venereally infected with Treponema paraluis-cuniculi between five and eight months previously, had treponeme-containing genital lesions and positive results to serological tests for syphilis. These rabbits and four normal female rabbits were challenged with Treponema pallidum intradermally on the shaved back with triplicate doses of 10(4), 10(3), 10(2), and 10 treponemes (all at different sites). Significantly fewer syphilitic lesions developed in the rabbits previously infected with T paraluis-cuniculi (4/36) compared with the control rabbits (37/48), and the mean size of the lesions (diameter of induration) was significantly less (4 mm compared with 10 mm). It appears that previous venereal infection of female rabbits with T paraluis-cuniculi induced a level of cross-immunity against infection with T pallidum. Protection was however not complete.