Recent reports postulate that the dual oxidase (DUOX) proteins function as part of a multicomponent oxidative pathway used by the respiratory mucosa to kill bacteria. The other components include epithelial ion transporters, which mediate the secretion of the oxidizable anion thiocyanate (SCN -) into airway surface liquid, and lactoperoxidase (LPO), which catalyzes the H 2O 2-dependent oxidation of the pseudohalide SCN - to yield the antimicrobial molecule hypothiocyanite (OSCN -). We hypothesized that this oxidative host defense system is also active against respiratory viruses. We evaluated the activity of oxidized LPO substrates against encapsidated and enveloped viruses. When tested for antiviral properties, the LPO-dependent production of OSCN - did not inactivate adenovirus or respiratory syncytial virus (RSV). However, substituting SCN - with the alternative LPO substrate iodide (I -) resulted in a marked reduction of both adenovirus transduction and RSV titer. Importantly, well-differentiated primary airway epithelia generated sufficient H 2O 2 to inactivate adenovirus or RSV when LPO and I - were supplied. The administration of a single dose of 130 mg of oral potassium iodide to human subjects increased serum I -concentrations, and resulted in the accumulation of I - in upper airway secretions. These results suggest that the LPO/I -/H 2O 2system can contribute to airway antiviral defenses. Furthermore, the delivery of I - to the airway mucosa may augment innate antiviral immunity.