Alpha-2a-interferon (IFN) has been shown to provide important local defense against some viral infections. In vitro, IFN inhibits the growth of respiratory syncytial virus in bovine monolayer cultures, but respiratory syncytial virus is known to be a poor inducer of IFN in infected infants. When IFN was administered by daily intramuscular injection in a double blind, placebo-controlled study of 22 infants with bronchiolitis caused by respiratory syncytial virus, there was no statistically significant difference in clinical course, duration of oxygen requirement or physical assessment between the treatment and control groups. Likewise viral isolation showed no difference between the groups. Further studies are needed to determine whether higher dosing or aerosol or other alternative route of delivery of IFN might lead to therapeutic benefit.