A prospective study of the efficacy of ampicillin in combination with sulbactam, a beta-lactamase inhibitor, (A/S) in perioperative prophylaxis was performed. The study consisted of two independent parts performed at the same time. Part I included 60 patients with lobectomies and segmentectomies. Group A (A/S 1 x 3 g "single shot") was compared with group B (A/S 3 x 3 g). Superficial wound infections occurred in 3 patients of group A and in 2 patients of group B. There was no empyema. Bronchitis and pneumonia were found in 10 patients of group A and in 7 patients of group B. Part II examined 25 pneumonectomies receiving A/S 3 x 3 g for 3 days. Concentrations of ampicillin and sulbactam in serum and lung tissue were determined and showed adequate levels to cope with usual bacteria in lung surgery. There was one superficial wound infection, 2 cases of bronchitis, and 2 cases of pneumonia.