From our study on cefuzonam (CZON), a new antibiotic agent, with regard to pulmonary tissue uptake and use in prophylaxis of postoperative infections in 20 patients undergoing thoracotomy, we were able to reach the following conclusions: 1. Following intravenous infusion over 1 hour of 1 g of CZON immediate preoperatively, the mean serum concentration at 1 hour after commencement of the infusion reached at 41.5 micrograms/ml (serum peak value), then gradually decreased with a half-life in the beta-phase of 1.20 hours. 2. The mean CZON concentration in normal pulmonary tissue at 1 hour after commencement of the infusion was 12.7 micrograms/g with a half-life in the beta-phase of 1.28 hours. Although the number of cases examined was small at only 4, the mean pulmonary tissue level in obstructive pneumonitic lesions was 13.7 micrograms/g, and an average half-life in the beta-phase was 1.65 hours. 3. Mean bronchiolar CZON concentrations at 2, 4, and 6 hours after commencement of the infusion were 14.5, 3.1, and 2.8 micrograms/g, respectively, with a half-life in the beta-phase of 3.53 hours. 4. Among the patients undergone thoracotomy with use of CZON, no postoperative infections occurred nor severe adverse reaction was observed. CZON attained good transfer in the pulmonary tissues, particularly in the bronchiolar tissue, hence it appears useful as a prophylactic against postoperative infections in respiratory disease.