DTPA absorption in the cystic fibrosis airway is normalized by an apical osmotic gradient. Apical DTPA absorption was measured in untreated CF epithelial cells and cells with 300 mM mannitol added to the apical surface (three CF cell lines, n = 6 cultures per line). Average data from seven non-CF lines presented for comparison purposes. Asterisk, p < 0.0001, comparing CF with treated CF. Each data point is mean ± SEM.