Repeated culture surveys were made on 13 patients undergoing major elective surgery. All patients had an endotracheal tube in place during operation only, and all used sonic form of respiratory assistance in the postoperative period. Ten patients received prophylactic antibiotics.Five patients had gram-negative bacilli in the preoperative nose, throat, and sputum cultures; and 11 of the 13 had these organisms present in the respiratory flora at some lime in the postoperative period. The cause of the unusually high incidence of these organisms may be related to illness, hospital environment, endotracheal intubation, surgery, or the use of systemic antibiotics. This frequency of respiratory colonization may be one factor contributing to the general increase in gram-negative sepsis.