Respiratory syncytial virus (RSV) antigen was demonstrated in formalin-fixed, paraffin-embedded autopsy tissue using an immunoperoxidase technique. Eighteen autopsy cases were selected on the basis of one of the following criteria: a positive culture for RSV, antemortem or postmortem; positive ELISA test for RSV, antemortem or postmortem; or postmortem histology suggestive of paramyxovirus infection. Controls included three cases from which parainfluenza or influenza virus had been cultured and a case in which the clinical diagnosis of measles was firmly established. Sections of formalin-fixed, paraffin-embedded tissue were stained with a rabbit anti-RSV antibody (Dako) using an immunoperoxidase technique. Staining was achieved in 12 cases. This included 6 of 7 cases selected because of positive cultures or ELISA tests for RSV. The other 6 cases in which RSV was identified by the described technique lacked culture or ELISA confirmation. Granular and globular staining was seen in the cytoplasm of respiratory epithelial cells and syncytial giant cells. None of the control cases stained for RSV. The histology of RSV lungs was consistent with changes described in the literature for RSV infection, although pneumonic consolidation and syncytial giant cells were more prominent in this series.