Increased activity of lung epithelial sodium channels (ENaCs) contributes to the pathophysiology of cystic fibrosis (CF) by increasing the rate of epithelial lining fluid reabsorption. Intera-inhibitor (IαI), a serum protease inhibitor, may decrease ENaC activity by preventing its cleavage by serine proteases. High concentrations of IαI were detected in the bronchoalveolar lavage fluid (BALF) of children with CF and lower airway diseases. IαI decreased amiloride-sensitive (IENaC) but not cAMP-activated Cl2 currents across confluent monolayers of rat ATII, and mouse nasal epithelial cells grew in primary culture by 45 and 25%, respectively. Changes in IENaC by IαI in ATII cells were accompanied by increased levels of uncleaved (immature) surface a-ENaC. IαI increased airway surface liquid depth overlying murine nasal epithelial cells to the same extent as amiloride, consistent with ENaC inhibition. Incubation of lung slices from C57BL/6, those lacking phenylalanine at position 508 (ΔF508), or CF transmembrane conductance regulator knockout mice with IαI for 3 hours decreased the open probability of their ENaC channels by 50%. ΔF508 mice had considerably higher levels the amiloride-sensitive fractions of ENaC nasal potential difference (ENaC-NPD) than wild-type littermates and only background levels of IαI in their BALF. A single intranasal instillation of IαI decreased their ENaC-NPD 24 hours later by 25%. In conclusion, we show that IαI is present in the BALF of children with CF, is an effective inhibitor of ENaC proteolysis, and decreases ENaC activity in lung epithelial cells of ΔF508 mice. Copyright © 2014 by the American Thoracic Society.