The presence of multinucleated giant cells has often been observed in the lungs of patients dying from severe lung infections. Such multinucleated cells were found in sections of lung tissue obtained at necropsy from 8 children dying within a period of 10 years in the Children's Hospital, Sheffield, from acute respiratory infections. Blocks of such tissue, fixed in formaldehyde-saline and embedded in paraffin wax, were available from these patients and were examined by a two-stage immunofluorescent staining technique for the presence of respiratory syncytial virus antigen. Specific fluorescence was seen in all 8 cases in a large proportion of the cells lining the bronchi including the characteristic multinucleated cells Control sections from 8 children [dying from road accidents (2), extensive burns (2) and primary lung infections (4)] did not show these multinucleated cells and did not show specific fluorescence. Flying cover-slip preparations, infected with respiratory syncytial virus and fixed in formaldehyde-saline, showed specific fluorescence for periods of up to 112 days, RNP Sutton.