The ability of macrophages to process and present treponemal antigens to T-lymphocytes was studied in early stages of experimental syphilis produced by Treponema pallidum Bosnia A strain (the causative agent of endemic syphilis) infection of inbred Syrian hamsters (LSH/Ss Lak strain). A difference was noticed in the response of macrophages obtained from the peritoneal cavity, lymph nodes, and spleens of the infected animals. In all of these locations, a general increase in the population of Ia(k)-positive macrophage was seen during the entire period of infection, i.e., 3 to 18 weeks after inoculation. Peritoneal cavity-derived macrophages showed no difference in antigen presentation to sensitized and nonsensitized T-lymphocytes for the first 7 weeks of infection. However, at 18 weeks after infection, peritoneal macrophages lost their ability to process treponema antigens. Spleen- and lymph node-derived macrophages did not exhibit a parallel loss in their ability to process treponema antigens. A fluctuation without a consistent pattern was noticed in the antigen processing and presentation by macrophages from the spleen and lymph nodes. In general, the sensitized T-lymphocytes responded to treponema antigen presented by macrophages more vigorously than the nonsensitized T-lymphocytes. An increased ability of spleen-derived macrophages to process and present antigens was noticed throughout the entire period of infection. The macrophages from the lymph nodes showed such an increase only temporarily at 3 weeks after infection. These data suggest that the processing and presentation of treponema antigens by macrophages in acute syphilitic infection fluctuates considerably and depends on the source of macrophages and the duration of the infection. The differences in the response of peritoneal cavity-, spleen-, and lymph node-derived macrophages probably reflect the complex interactions between the macrophage and other cells involved in the immune response to treponema infection.