Human leucocytary interferon (IFN) in a concentration of 2,000 Ul/ml stops the development of 9 Rhinovirus (RV) strains. The use of serum IFN antibodies (SAIF) reverses this effect. Of 256 nasal aspirates culture negative for other respiratory viruses, 40 RV were isolated with SAIF as opposed to 19 without serum anti IFN. The frequency and some clinical aspects are emphasized in infants: 43% had bronchiolitis similar to respiratory syncytial virus (RSV) infections and atopy was frequent.