Treponema pallidum partially inhibited the synthesis of DNA, RNA, and protein by rabbit cells in vitro. The inhibition of DNA synthesis was proportional to treponemal concentration and persisted during the period of exposure to T. pallidum. The toxic effect was not dependent on treponemal metabolism or on whole treponemes, since heat- and penicillin-killed treponemes and a cell-free sonicate of treponemes had similar toxicities. The toxic factor(s) was also detected in extracts of syphilitic rabbit testes but not in extracts of normal rabbit testes or testes inflamed by chemical means. The T. pallidum-derived toxic material had a molecular weight greater than 20,000 as determined by dialysis. Protein and DNA synthesis were most rapidly inhibited; RNA synthesis continued at normal rates for up to 2 h after exposure to treponemes. Protein synthesis or a necessary precursor of protein synthesis appeared to be the primary target of the T. pallidum toxin(s).