Reduced terminal sialylation at the surface of airway epithelial cells from patients with cystic fibrosis may predispose them to bacterial infection. To determine whether a lack of chloride transport or misprocessing of mutant cystic fibrosis transmembrane conductance regulator (CFTR) is critical for the alterations in glycosylation, we studied a normal human tracheal epithelial cell line (9/HTEo⁻) transfected with the regulatory (R) domain of CFTR, which blocks CFTR-mediated chloride transport; ΔF508 CFTR, which is misprocessed, wild-type CFTR; or empty vector. Reduced cAMP-stimulated chloride transport is seen in the R domain and ΔF508 transfectants. These two cell lines had consistent, significantly reduced binding of elderberry bark lectin, which recognizes terminal sialic acid in the α-2,6 configuration. Binding of other lectins, including Maakia amurensis lectin, which recognizes sialic acid in the α-2,3 configuration, was comparable in all cell lines. Because the cell surface change occurred in R domain-transfected cells, which continue to express wild-type CFTR, it cannot be related entirely to misprocessed or overexpressed CFTR. It is associated most closely with reduced CFTR activity.