Cefmenoxime (CMX, SCE-1365), a newly synthesized cephalosporins by Takeda Chemical Industries Ltd., Japan, was administered to 18 peritonitis cases, and its efficacy and safety was studied. The major part of a route of administration was i.v. drip infusion and dose was 1 to 4 g daily. Isolation of the causative organisms was made from the exudate of drain and clinical effectiveness was evaluated by the doctors in consideration of the effect on causative organism, body temperature, white blood cells count, state of discharge and etc. The majority of causative organisms were gram-negative rods and anaerobes, and bacteriological eradication rate was 85%. Regarding clinical efficacy, 5 cases (38%) responcied to Cefmenoxime treatment excellently, 6 cases (46%) effectively and 2 cases (15%) fairly, and no poorly responded cases were found. Five cases received other antibiotics concomitantly were deleted from efficacy evalation. As for side effects, drug eruption was noted in one case while the causality was doubtful according to doctor's opinion. Regarding abnormal change of laboratory findings, elevation of GOT and GPT values was noted in one case which returned to normal after the discontinuation of the administration. Thus, cefmenoxime is considered to be the useful cephalosporins for the treatment of peritonitis from the view point of safety and efficacy.