Seventy-eight unselected children under the age of 2 years suffering from acute respiratory infections were investigated by the fluorescent antibody technique and a comparison was made with conventional isolation and serological methods. Sixty-nine per cent. of children with bronchiolitis were diagnosed as suffering from respiratory syncytial virus infection on the day of admission by examination of nasopharyngeal secretions. There were 44 children with respiratory syncytial virus infection diagnosed by conventional methods over a month, but by using fluorescent antibody techniques on tissue culture 53% were diagnosed by the second day, 71% by the fourth day, and 82% by the seventh day. The method of choice for a rapid diagnosis of respiratory syncytial virus infection is by examining nasopharyngeal secretions, when 90% of those with this infection can be diagnosed on the day of admission to hospital.