A method to determine changes in the activity of natural killers (NK) in treatment of juvenile respiratory papillomatosis in children and correlation with the clinical course of the disease was developed and studies with whole blood specimens were carried out before interferon (IF) treatment and after it. Initially high levels of NK activity and their subsequent decline were observed in the cases where IF treatment had no effect, whereas initially low levels with subsequent increase after treatment or normal levels remaining unchanged in the course of treatment were typical of the patients manifesting some clinical effect of the treatment. These studies confirm the possibility of the modulating effect of IF, and the method may be used in mass surveys and working with children.