T o test whether a major contribution of airways epithelial ion transport to lung defense reflects the regulation of airway surface liquid (ASL) ionic composition, we measured ASL composition using the filter paper technique. On nasal surfaces, the Cl- concentration (approximately 125 meq/liter) was similar to plasma, but the Na+ concentration (approximately 110 meq/liter) was below plasma, and K+ concentration (approximately 30 meq/liter) above plasma. The resting ASL osmolarity [2(Na+ + K+); 277 meq/liter] approximated isotonicity. There were no detectable differences between cystic fibrosis (CF) and normal subjects. In the lower airways, the Na+ concentrations were 80-85 meq/liter, K+ levels approximately 15 meq/liter, and Cl- concentrations 75-80 meq/liter. Measurements of Na+ activity with Na(+)-selective electrodes and osmolality with freezing point depression yielded values consistent with the monovalent cation measurements. Like the nasal surfaces, no differences in cations were detected between CF, normal, or chronic bronchitis subjects. The tracheobronchial ASL hypotonicity was hypothesized to reflect collection-induced gland secretion, a speculation consistent with observations in which induction of nasal gland secretion produced hypotonic secretions. We conclude that there are no significant differences in ASL ion concentrations between CF, normal, and chronic bronchitis subjects and, because ASL ion concentrations exceed values consistent with defensin activity, the failure of CF lung defense may reflect predominantly factors other than salt-dependent defensins.