Gene transfer could provide a novel treatment for cystic fibrosis. However, current vectors, including recombinant adenoviruses, are relatively inefficient at gene transfer to airway epithelia. We have found that delivering adenovirus in a calcium phosphate coprecipitate (Ad:CaPi coprecipitates) enhanced the efficiency of gene transfer to airway epithelia in vitro and in vivo . However, the potential for injury to the epithelium was not evaluated. In NIH 3T3 cells treated with Ad:CaPi coprecipitates, we found that a 30-min exposure, which was sufficient for maximal transgene expression, produced no toxicity; whereas some other transfection reagents induced significant toxicity. Moreover, when Ad:CaPi coprecipitates were applied to the apical surface of differentiated airway epithelia in vitro, they did not reduce transepithelial resistance, even after prolonged incubation. Delivery of Ad:CaPi coprecipitates to mouse lung induced an inflammatory response, but it was not substantially different from that following administration of adenovirus alone. Thus, Ad:CaPi coprecipitates significantly enhance gene transfer to differentiated human airway epithelia in vitro and to mouse lung in vivo without increasing toxicity or the inflammatory response. Thus, CaPi coprecipitates may enhance the therapeutic index of adenovirus-based gene transfer vectors.