Pulmonary epithelial cells are thought to be the primary cellular targets for infection by respiratory syncytial virus (RSV) in vivo. To determine whether other pulmonary cells are infected by RSV, bronchoalveolar lavage cells from six adult transplant patients, four of whom had acute RSV infection, were examined by in situ immunohistochemistry to identify infected lung cells. Both alveolar macrophages and epithelial cells were infected with RSV in vivo. At the single-cell level, three-color immunofluorescent studies revealed that both RSV-infected epithelial cells and alveolar macrophages expressed Class II molecules of the major histocompatibility complex, but only the alveolar macrophage coexpressed interleukin-1β. Paraformaldehyde- fixed bronchoalveolar lavage cells from RSV-infected but not uninfected patients induced a marked proliferative response by cloned T cells indicating that in vivo infected cells expressed bioactive interleukin-1. Together, these studies indicate that the alveolar macrophage may have a critical role in the lung immune response to RSV.