A virus with the characteristics of the respiratory syncytial virus was isolated from the throat of a 6-month-old infant with pneumonia. The illness was accompanied by an eightfold increase in complement-fixation antibody to the Long strain of the respiratory syncytial virus and a sixteen-fold rise in the homologous neutralizing antibody, indicating that the pneumonia was accompanied by infection with this virus. The relationship between this infection and the patient's pneumonia is possibly etiologic.