Although fluoroquinolone antibiotics such as ciprofloxacin are able to gain access to lung tissue and both pleural and bronchial secretions, the characteristics of transport and cellular uptake of ciprofloxacin in human epithelial lung tissue remain obscure. We have chosen human airway epithelial (Calu-3) cells, reconstituted as functional epithelial layers grown on permeable filter supports, as a model with which to assess both transepithelial transport and cellular uptake of ciprofloxacin. Transepithelial ciprofloxacin fluxes in absorptive (apical-to-basal) and secretory (basal-to-apical) directions were similar throughout the concentration range studied (1.0 microM to 3.0 mM). Transepithelial mannitol fluxes measured concurrently were substantially smaller than ciprofloxacin fluxes in Calu-3 epithelia, suggesting the existence of a mediated transcellular route in addition to a paracellular route for transepithelial permeation. Apical-to-basal ciprofloxacin flux (at 10 microM) was inhibited by a 100-fold excess of unlabelled norfloxacin, enoxacin, and ofloxacin, while secretory flux was unaffected. Cellular uptake of ciprofloxacin, determined as a cell/medium ratio, was greater from the basal compartment (2.7-fold) than apical uptake (1.39-fold) measured at 100 microM ciprofloxacin and showed no saturation up to 3 mM ciprofloxacin. Comparison of the permeation of ciprofloxacin was made with that of lipophilic substrates such as vinblastine and digoxin. There was a linear correlation between transepithelial permeability (Pa-b) and their oil/water partition coefficients with mannitol < ciprofloxacin < digoxin < vinblastine. Comparison of transport of ciprofloxacin across human airway Calu-3 epithelia with that across intestinal Caco-2 epithelia emphasizes the absence of a specific secretory pathway; ciprofloxacin permeation in Calu-3 epithelia appears to be mediated primarily by a transcellular route, with mediated transfer at apical and basal membranes occurring via transporters with low affinity to ciprofloxacin.