Efficacy of intranasal recombinant alpha interferon (IFN-alpha 2b) was evaluated over a four-week period. The first 400 participants received either 1,500,000 IU of IFN-alpha 2b or placebo twice daily. Rhinovirus infections were prevented (protective efficacy, 76%). Parainfluenza infections were not prevented, but symptoms in associated episodes of disease were significantly reduced. The medication was generally well tolerated, but side effects were often observed. The most commonly reported symptom was blood-tinged mucus. A pilot study of IFN-alpha 2b or placebo administered on a once-daily dose schedule was also carried out in 150 participants. There was a suggestion of continued efficacy with reduced side effects. Overall, these findings would limit the use of IFN-alpha 2b on the twice-daily schedule to shorter time periods or to special situations in which the efficacy clearly outweighs side effects, and they encourage further examination of other dosage schedules.