Superantigen hyperactivation of the immune system has variable, sometimes lethal consequences for the host. Here we show that concurrent influenza infection enhanced the effects of the bacterial superantigen staphylococcal enterotoxin B (SEB) in mice. The effect was T cell-dependent, and maximal synergism was observed when SEB was administered 7 days after the virus, a timepoint during infection associated with high viral titers, a vigorous cytotoxic T cell response, and extensive lung pathology. The influenza infection enhanced the SEB-induced cytokine response in terms of higher absolute levels of cytokine, sustained secretion, and localization to the respiratory tract. In particular, TNF and IFN-gamma were implicated in the mechanism of death because their neutralization protected the mice from death, and recombinant IFN-gamma and TNFalpha mimicked the lethal effect of SEB in influenza-infected mice. This lethal synergism between concurrent influenza infection and superantigen exposure points to the danger of secondary bacterial involvement in viral pneumonia, and suggests mechanisms that may contribute to sudden and unexpected death from influenza infection. In addition, these data demonstrate that the in vivo effects of superantigen exposure can be strongly influenced by the immune activation status of the host.