SCN−(thiocyanate) is an important physiological anion involved in innate defense of mucosal surfaces. SCN− is oxidized by H 2 O 2, a reaction catalyzed by lactoperoxidase, to produce OSCN−(hypothiocyanite), a molecule with antimicrobial activity. Given the importance of the availability of SCN− in the airway surface fluid, we studied transepithelial SCN− transport in the human bronchial epithelium. We found evidence for at least three mechanisms for basolateral to apical SCN− flux. cAMP and Ca 2+ regulatory pathways controlled SCN− transport through cystic fibrosis transmembrane conductance regulator and Ca 2+-activated Cl− channels, respectively, the latter mechanism being significantly increased by treatment with IL-4. Stimulation with IL-4 also induced the strong up-regulation of an electroneutral SCN−/Cl− exchange. Global gene expression analysis with microarrays and functional studies indicated pendrin (SLC26A4) as the protein responsible for this SCN− transport. Measurements of H 2 O 2 production at the apical surface of bronchial cells indicated that the extent of SCN− transport is important to modulate the conversion of this oxidant molecule by the lactoperoxidase system. Our studies indicate that the human bronchial epithelium expresses various SCN− transport mechanisms under resting and stimulated conditions. Defects in SCN− transport in the airways may be responsible for susceptibility to infections and/or decreased ability to scavenge oxidants.