Children with viral respiratory infections and healthy contacts received treatment and prophylaxis with leukocytic interferon. The results emphasize the effectiveness of treatment when interferon is given on the first or second day of the disease. In these cases the course is mild, of short duration, and without complications. Conversely, the clinical course in children who did not receive interferon was more severe; the disease lasted longer, and complications were not uncommon. Prophylactic instillation of interferon succeded in protecting 85.2 percent of contacts; those who contracted the infection had a mild disease with insignificant complications. No allergic or intoxication phenomena were observed in children receiving leukocytic interferon for treatment or prophylaxis.