The bacterial exotoxins staphylococcal enterotoxin A and B (SEA and SEB) mediate disease through their effects on T lymphocytes. In this manuscript we have demonstrated that both SEA and SEB can directly activate purified T cells in the absence of accessory cells as determined by a transition from G0 to G1 and induction of IL-2 receptor expression. However, neither SEA nor SEB alone was sufficient to result in T-cell proliferation. The induction of T-cell proliferation by SEB or SEA required the addition of a second costimulatory signal. This could be provided by either accessory cells or monoclonal antibody stimulation of CD28. As previously reported, T-cell proliferation induced by enterotoxin in the presence of accessory cells was partially inhibited by a blocking antibody against class II MHC. In contrast, in purified T cells when costimulation was provided through CD28, proliferation was not inhibited by class II antibody, and HLA-DR expression was not detectable. In addition, costimulation through CD28 was partially resistant to the effects of cyclosporin A. These results demonstrate that CD28 costimulation is sufficient to induce proliferation of enterotoxin-activated T cells, and that this effect is independent of class II MHC expression.