Three outbreaks of influenza caused by influenza A H1N1 occurred in a boys' boarding school in 1978, 1979 and 1983. The serological response to infection with variants of the H1N1 virus was studied by radial haemolysis and haemagglutination inhibition after primary infection and reinfection. The persistence of this antibody was also studied. Infection in 1978 resulted in the production of persistent antibody to both the haemagglutinin and neuraminidase of the homotypic strain. Antibody which cross-reacted with later variants of the virus was less frequently produced, the peak response was delayed and such antibody persisted less well. Infection in 1979 resulted in a similar response to that observed in 1978 after primary infection. Reinfection resulted in a broad response in all cases. In 1983 all infected boys produced antibody which reacted with the homotypic strain but only approximately one-third of primary infections produced antibody which reacted with the A/USSR/92/77 strain. The neuraminidase of the A/USSR strain failed to detect one third of the primary infections. Reinfection again resulted in a broad response.