Serum antibody responses to primary and secondary infections with respiratory syncytial virus were determined in 67 hospitalized infants. Responses in IgG, IgM, and IgA immunoglobulin fractions were assayed using an indirect immunofluorescent technique. Infection was confirmed by identification of RSV antigen in nasopharyngeal secretions using indirect immunofluorescence, and by recovery of the virus in tissue culture. IgM specific antibody response was commonly observed within a few days of onset of primary infection and persisted from two to ten weeks. Increases in titer of virus-specific IgG and IgA were observed in infants aged 6 months or more at the time of infection than in younger infants. At one year after primary infection, IgA and IgM specific antibody was undetectable, and IgG antibody was absent or present in low titer. Subsequent reinfection result ed in accelerated antibody response in all three classes of serum immunoglobulin. High titers of RSV-specific IgG, IgM, and IgA antibody were detectable at five and seven days after reinfection. These observations may explain the relatively mild nature of clinical illness associated with RSV reinfections in childhood.