The capability of bovine polymorphonuclear (PMN) and mononuclear phagocytes to kill Campylobacter (Vibrio) fetus venerealis was tested under various conditions in order to judge their roles in protective immunity. Bovine PMN killed C fetus in the presence of immunoglobulin G (IgG) but not immunoglobulin A (IgA) antibodies. Some phagocytosis occurred in the absence of antibody in glass-adherent cultures, but not in suspension cultures--indicating that surface phagocytosis by the neutrophils normally present during estrus could account for natural protection at this time. Mononuclear phagocytes also killed C fetus in the presence of opsonins, making this a likely factor in protection during the later stages of inflammation when mononuclear cells predominate. The presence of numerous lymphocytes in this mononuclear infiltration and the capability of C fetus antigens to induce delayed hypersensitivity (DH) are consistent with the possibility that cell-mediated immunity (CMI) also may be involved in protective immunity.