From December 1985 to February 1986 in Si Hong County, Jiang Su Province an outbreak of bronchopneumonitis occurred. In December the number of admitted infant cases with bronchopneumonitis was 32% of total hospitalized cases. The youngest infant was 28 days in age and the oldest was one and half years. 71.5% percent occurred in children less than six months in age. From nasopharyngeal washing of acute cases were isolated 6 strains of viruses. The result of identification of them revealed that they were respiratory syncytial virus. 3 pairs of serum sample were collected from acute and convalescent periods for neutralizing antibody titrations. The result showed that specific neutralizing antibody titers increased 8-64 fold in comparison between the acute and the convalescent.