T cells appear to play a central role in viral bronchiolitis, but the effects of different functional and phenotypic subgroups of T cells have not been defined. To test the activities of T cells recognizing individual proteins of respiratory syncytial (RS) virus, virus-specific T cell lines were produced from mice primed by scarification with recombinant vaccinia viruses expressing the major surface glycoprotein (G), fusion protein (F) or second matrix (22K) protein of RS virus. As previously reported, the in vitro characteristics of these cells are predetermined by the choice of RS virus protein: 22K-specific cells are predominantly class I-restricted cytolytic CD8+ cells; F-specific cells, a mixture of cytolytic CD8+ cells and CD4+ cells with a T helper 1 cell (Th1) cytokine secretion profile, whereas those from G-sensitized mice are almost exclusively CD4+, with Th2 characteristics. Mice infected intranasally with RS virus showed mild illness and recovered fully, but developed respiratory distress after intravenous injections of T cells. Dose-for-dose, infected mice receiving G-specific cells suffered the most severe (sometimes fatal) illness, characterized by lung hemorrhage, pulmonary neutrophil recruitment (shock lung) and intense pulmonary eosinophilia. This disease was further enhanced by coinjection of 22K-specific cells, which alone caused mild shock lung without eosinophilia. F-specific cells caused minimal enhancement of pathology and had little or no effect on the disease caused by G-specific cells. Each cell line reduced lung virus titer and combined injections of G- and 22K-specific cells eliminated infection completely. The in vitro characteristics of these antiviral T cell lines therefore predict the pathological effects in vivo. Moreover, different forms of viral bronchiolitis can be caused by functionally distinct types of activated T cell.