Interferon levels in nasal secretions of infants under one year of age, and hospitalized with lower repiratory tract disease, were measured during two respiratory infection seasons. In the first year serial secretions from 50 infants with respiratory syncytial virus infection were examined. Undetectable or low levels of interferon were found in all samples, and mean levels did not fluctuate significantly in relation to disease and recovery. This was in contrast to anti-RSV IgA, which appeared and increased in concentration as virus shedding decreased and stopped. In the second year secretions were obtained from nine infants with influenza A virus infection as well as from 13 with RSV. All those with influenza developed measurable interferon in secretions (geometric mean titer 138 units/ml), which was acid and heat stable, and trypsin sensitive (type I interferon). RSV infection again stimulated very low levels (geometric mean 5 units/ml). The lack of correlation of interferon concentration with cessation of RSV shedding suggests either that it is not involved in recovery or that low levels are adequate. On the other hand, it appears that the young infant is fully capable of a brisk local interferon response, at least to infection by influenza A.