Intranasal sprays of interferons (IFNs) given one day before and for three days after virus challenge can protect human volunteers from infection with rhinoviruses, coronavirus, and influenza. Longer dosage of IFN gives rise to nasal symptoms and signs such as bloodstained nasal discharge. More effective IFNs and regimes are therefore needed. IFN beta is active but the degree to which it will irritate the nose is unknown. Combining IFNs with synthetic antiviral drugs can produce synergistic increases in antiviral activity. It is suggested that these increases may be exploited in future experiments.