The entry of ampicillin, cephalothin and gentamicin into traceobronchopulmonary secretions/exudates was assessed in 22 patients during 28, episodes of pneumonia or bronchitis. Specimens were collected from the lower respiratory tract via tracheostomies or endotracheal tubes using either the flexible fiberoptic bronchoscope (50 specimens) or an intratracheal catheter (59 specimens). Venous blood was obtained at the same time. The concentrations in the bronchial specimens were less than those in the corresponding serums, amounting to about 10 per cent with ampicillin, 25 per cent with cephalothin and equal to or greater than 40 per cent with gentamicin.