Calu-3 cells, a human airway epithelial cell line (ATCC, Manassas, VA), were seeded on collagen-coated MillicellH-PCF membrane inserts (Millipore, Billerica, MA) at a density of 1610 6 cells per insert of 0.6 cm 2 surface area. Two days after the initial submerged culture, the apical media were aspirated off and the cells cultured at an air-liquid interface according to previously published protocol [23] . Regardless of submerged culture or air-liquid interface culture, the media used were the same, consisting of Advanced-MEM (Gibco, Carlsbad, CA) containing 10% fetal bovine serum, 1% L-glutamine, 100 U/ml penicillin, 100 mg/ml streptomycin and 0.25 mg/ml amphotericin B. After 2 weeks at 37uC in presence of 5% CO 2 , the epithelia established a dry apical surface and had a transepithelial electrical resistance greater than 1000 V/cm 2 . The cystic fibrosis (CF) airway epithelial cells CFBE41o- [24] were similarly cultured. The fully differentiated CF epithelia after 2 weeks exhibited a transepithelial resistance greater than 700 V/cm 2 .