HLA class II molecules present superantigens more efficiently than their murine counterpart. Therefore, transgenic mice expressing HLA‐DQ8 with and without CD28 were used to address the role of CD28 in staphylococcal enterotoxin B (SEB)‐driven immune responses. SEB‐induced in vitro proliferation of naive DQ8.CD28^–/– splenocytes was comparable to DQ8.CD28^+/+ cells, and was several fold higher than that of C57BL/10 and BALB/c splenocytes. SEB‐activated, naive DQ8.CD28^–/– cells in vitro produced significantly less IL‐2, IL‐4 and IL‐10 than DQ8.CD28^+/+ cells, while IFN‐γ and IL‐6 production was comparable. SEB‐induced in vivo expansion of CD4⁺ T cells and, to a greater extent, CD8⁺ T cells was compromised in DQ8.CD28^–/– mice, indicating that SEB‐induced proliferation of CD8⁺ T cells is more dependent on CD28 co‐stimulation. Upon re‐stimulation, SEB‐primed CD28^+/+ T cells failed to proliferate but were capable of producing cytokines. Conversely, CD28^–/– T cells were capable of proliferation, but not cytokine production. SEB‐primed CD28‐deficient cells produced significantly less nitric oxide when compared to CD28‐sufficient cells following re‐stimulation with SEB. CD28^+/+ and not CD28^–/– mice were highly susceptible to SEB‐induced lethal shock characterized by significantly elevated serum IFN‐γ. Thus, (i) efficient presentation of SEB by HLA‐DQ8 circumvents co‐stimulation through CD28, (ii) unique CD28‐derived signals are mandatory for generation of certain effector functions, and (iii) absence of CD28‐derived signals confers resistance to activation‐induced cell death and protects mice from SEB‐induced shock.