Toxic shock syndrome toxin (TSST)-1 is a superantigen known to profoundly induce proinflammatory cytokines by activation of V beta -specific alpha beta T cells, but its effect on gamma delta T cells, which normally constitute 1%-5% of peripheral blood mononuclear cells (PBMCs), is unclear. Here, we demonstrate that TSST-1 induced significantly higher levels of interferon (IFN)- gamma, tumor necrosis factor (TNF)- alpha, and interleukin (IL)-2, and a lower level of IL-10 in human PBMCs when the gamma delta subpopulation has been primed by isopentylpyrophosphate, compared with that in control PBMCs. Furthermore, depletion of the gamma delta subpopulation completely abrogated this effect. Thus, peripheral gamma delta T cells markedly modulate both the proinflammatory and anti-inflammatory cytokine responses of TSST-1.