A study was made of the organisms cultured from the respiratory tract at or before operation and of the incidence of postoperative bronchopleural fistula, pneumonectomy space infection or chest infection in 221 consecutive patients undergoing thoracotomy. Patients received either no antibiotics or one of three variations of an ampicillin/flucloxacillin mixture. Prophylactic systemic antibiotics significantly reduced the number of pneumonectomy space and chest infections; application of penicillin plus streptomycin powder to the bronchial stump had no effect. Systemic antibiotics given preoperatively reduced the incidence of bronchopleural fistula. The type of bacteria present in the preoperative sputum or bronchial lumen at operation was no help in predicting the effective therapy of postoperative infection. © 1981.