CD8+ T cells down-regulate a variety of immune responses. For example, porcine and human insulin do not stimulate Abs in C57BL/6 mice because CD8+ T cells inhibit CD4+ helper T cells. By contrast, bovine insulin induces Ab in C57BL/6 mice, and removal of CD8+ T cells does not alter this response. This raises the question of whether porcine, but not bovine, insulin activates CD8+ T cells or whether both insulins activate CD8+ T cells but CD4+ helper T cells are differentially inhibited by them. In this study, we show that insulin-specific CD8+ CTL can be cultured from C57BL/6 mice primed with either bovine or human insulin in CFA. Thus, exogenous Ags, besides OVA, induce CD8+ CTL when administered in an adjuvant, suggesting this is a typical response. These CTL are H-2Kb restricted and produce IL-5, IL-10, IFN-γ, and small amounts of IL-4, which is distinct from IFN-γ and TNF-α that are typically secreted by virus-specific CTL. Moreover, the CTL primed with either bovine or human insulin recognize an A-chain peptide that is identical to the mouse insulin sequence. That foreign proteins, which are closely related to self-proteins, activated autoreactive, CD8+ T cells in vivo is a novel finding. It raises the possibility that self-reactive CTL may be activated by cross-reacting Ags and once activated they might participate in autoimmunity. These results also suggest that down-regulation of insulin- specific responses by autoreactive CD8+ T cells is most likely due to the differential sensitivity of bovine and human insulin-specific CD4+ T cells.