Children of school age experience the greatest morbidity in influenza outbreaks and are major disseminators of the virus. On this basis, monovalent A2/Aichi/2/68 vaccine was offered to the schoolchildren of Tecumseh, Michigan, to control the anticipated outbreak of Hong Kong influenza. More than 85% of the children were vaccinated. Systemic reactions were rare and 94.5% of those tested exhibited a 4-fold or greater rise in HI titre.The occurrence of respiratory infections in the subsequent outbreak was followed in Tecumseh and the neighbouring unvaccinated community of Adrian, Michigan. Hong Kong influenza virus was isolated during a 10-week period, and, during this time the mean rate of illness in Adrian was 3.0 times higher than in Tecumseh. The protection from illness in Tecumseh was not limited to the vaccinated children; all age-groups experienced lower rates of respiratory infection. Thus vaccination of schoolchildren was shown to produce a marked lowering of illness rates in an entire community.