The ability of proteose peptone-induced normal rabbit peritoneal macrophages to kill Treponema pallidum subspecies pallidum in vitro is demonstrated. Treponemes and 10% heated immune or normal sera were incubated with macrophages at a ratio of 1:200. After 2-10 h of incubation, these mixtures were injected intradermally at duplicate sites on normal rabbits. Maximal killing (failure to develop lesions) was seen at 10 h of incubation with immune serum: Only 7% (1/14) of lesions developed compared with 90% (9/10) after incubation in the presence of normal serum (P less than .001). Maximal phagocytosis (detected by immunofluorescence) occurred by 8 h in the presence of immune serum, when 90% of macrophages had ingested treponemes. At this point, however, 70% of lesion sites from macrophages incubated with treponemes and immune serum still developed, suggesting that effective killing may require at least 2 h after phagocytosis.