A series of 87 controlled trials of the effectiveness of British gammaglobulin in preventing infectious hepatitis in schools and other institutions showed that gammaglobulin is very effective in these circumstances. No protection was given for the first two weeks after injection, probably because it was given during the incubation period of the hepatitis. The risks of developing hepatitis in contacts was found to vary greatly. In day schools there was usually little tendency for the disease to spread among the population, but the pupils in closest contact were at greater risk. In mental hospitals and children's homes, on the other hand, the tendency for the disease to spread was more pronounced.