Respiratory syncytial virus (RSV) is a frequent cause of severe lung disease in young children. Primed T cells are required for virus clearance, but are causally implicated in the enhanced pathology seen following RSV infection of some infants and experimental animals vaccinated against the virus. In BALB/c mice, vaccination with recombinant vaccinia virus expressing the viral attachment protein (G) leads to pulmonary eosinophilia during subsequent infection, which indirect evidence suggests may be due to CD4+ Th2 cells. The production of IFN-gamma, IL-2, -4, -5 and -10 cytokine mRNA by RT-PCR and intracellular cytokines by flow cytometry following RSV challenge of vaccinated mice were therefore compared. Lung eosinophilia was associated with enhanced local recruitment of CD4+ cells in G sensitized mice, while CD8+ cells dominated in mice vaccinated with the viral fusion protein (F) or second matrix protein (M2). Lung eosinophilia was also associated with a localized reduction in IFN-gamma and increased IL-4 and IL-5 mRNA transcription as well as elevated RSV specific IgG1 antibody production. Th2 cytokine protein production by T cells showed no apparent change. Although IFN-gamma production diminished in eosinophilic mice, it remained the major cytokine found in lung T cells. It was concluded that lung eosinophilia can develop despite abundant IFN-gamma production by local T cells, but is associated with a shift in the balance between Th2 and Th1 cytokine production.