The activity of exogenous human leukocyte IF has been studied in the human nasal mucosa and factors have been sought that would enhance its potential for clinical usefulness. Experiments have been performed which indicate that the nasal epithelial cells do respond to exogenous IF. In vivo dose requirements may be high because of mucociliary clearance mechanisms and because of the time and dose requirement for full antiviral activity. Future studies must define the IF sensitivity of different respiratory viruses in the human nasal epithelial cell and must evaluate more efficient methods of application.