The in vitro bactericidal activity of certain antimicrobials, especially aminoglycosides, is markedly diminished at an acid pH. In an attempt to correlate this factor with the poor response of gram-negative bacillary pneumonia to aminoglycoside therapy, the endobronchial pH of selected groups of subjects was measured via a bronchoscopically directed pH electrode. The average peripheral, i.e., subsegmental, endobronchial pH of normal persons, patients with chronic lung disease, and patients with pneumonia was 6.58 +/- 0.06, 6.62 +/- 0.10, and 6.61 +/- 0.06, respectively. The average central airway, i.e., major bronchi or tracheal, pH at 6.64 +/- 0.07 did not vary significantly from that of peripheral airways. The presence of pneumonia in individual bronchi was associated with a significantly lower pH than that in noninfected bronchi: 6.48 +/- 0.12 versus 6.69 +/- 0.13 (p less than 0.05). This relatively acid environment appears exaggerated within pneumonic airways and may contribute toward decreasing the effectiveness of aminoglycosides and certain other antimicrobials used in treating lung infection.