The authors studied the effect of antibiotics on Salmonella typhi Ty2 ingested by macrophages, which were obtained from the peritoneal cavities of two male rabbits. The applied antibiotics were ampicillin, chloramphenicol and bactrim. The antibiotic effect was determined with the viable bacterial numbers in the macrophages at 6, 12, 24, 36, 48 and 72 hour-cultivation. The results were summarized as follows : 1. Salmonella typhi Ty2 ingested by macrophages were slightly protected from the killing action of antibiotics than the same organisms not ingested by macrophages. 2. The killing action of ampicillin was slightly decreased, when it combined with other antibiotics. The synergic action of chloramphenicol and bactrim was not observed.