A combined agent of sulbactam, a beta-lactamase inhibitor, plus cefoperazone (SBT/CPZ: 1/1) was studied on the passage into the maternal milk as well on the clinical application and the results were followings: One or 2 grams of SBT/CPZ was given by one shot intravenous injection to puerperal mothers to determine the passage into the milk up to 6 hours after the administration. A trace of passage was detected for both SBT and CPZ as in the case of other antibiotics. Good clinical efficacy was observed in all 5 patients with female genital organ infections, to whom a daily dose of 2.0 g of SBT/CPZ was administered by drip infusion for 4 to 6 days. No side effects were noticed.