(A) Effect of azithromycin on TER of human airway epithelia in vitro. Human airway epithelial cells were cultured on Transwell filters. After reaching confluence, azithromycin (0.4, 4.0, and 40 μg/ml) was added to the basolateral side of epithelia every 48 h for 8 days. TER was measured using a Millicell-ERS electrical resistance system. Data are given as means ± SEM (n = 3). Azithromycin increased TER in a dose-dependent manner. (B) Azithromycin increases TER in human airway epithelia. Measurements were made at day 0 (open bars), before any treatment, and at day 4 (solid bars), after four doses of 40 μg/ml azithromycin. Data are given as means ± SEM (n = 6). Forty micrograms per milliliter of azithromycin added daily increased TER significantly (P < 0.0001). (C) Growth curve. Human airway epithelial cells were cultured on 24-well plates and treated continuously with 40 μg/ml azithromycin. Data are given as means ± SEM (n = 3). Azithromycin (40 μg/ml) decreases cell proliferation. AZM, azithromycin.