Fusobacterium nucleatum has been implicated in the pathogenesis of several diseases, including urinary tract infections, bacteremia, pericarditis, otitis media, and disorders of the oral cavity such as pulpal infections, alveolar bone abscesses, and periodontal disease. In this study, we examined sonic extracts of F. nucleatum strain FDC 364 for its ability to alter human lymphocyte function. We found that the soluble cytoplasmic fraction (CF) of the sonic extract was able to cause a dose-dependent inhibition of human lymphocyte responsiveness to Con A, PHA, PWM, and the recall antigen SKSD. Suppression involved altered DNA, RNA, and protein synthesis; there was no effect on cell viability. The suppressive activity is nondialyzable and heat labile. To achieve maximal suppression in 96-hr cell cultures, the CF had to be added to cells during the first 24 hr of incubation. Inhibition was reduced when the CF was added at 48 hr, and no suppression was observed when addition was at 72 or 96 hr (along with [3H]TdR). Furthermore, cells could be protected from the suppressive effects of the CF by washing within 24 hr of exposure. Suppression did not involve nonspecific effects on thymidine utilization. Although the mechanism of action of the F. nucleatum immunosuppressive activity has not yet been determined, we can rule out a requirement for monocytes/macrophages and activation of T suppressor cells. It has been proposed that impaired host defense may play a pivotal role in the pathogenesis of many diseases. The data presented in this paper suggest that local and/or systemic immunosuppression could be initiated by F. nucleatum. This immunosuppression may alter the nature and consequences of host-parasite interactions, thereby enhancing the pathogenicity of F. nucleatum itself or that of some other opportunistic organism.