The ability to distinguish self and non-self remains a central, though incompletely understood, prescript of immunology. In the absence of disease, the organism maintains immunological unresponsiveness, or tolerance, to self proteins. Unresponsiveness also extends to certain exogenous proteins, and unresponsiveness to many of these proteins is regulated by immune response (Ir) genes encoded by the major histocompatibility complex (MHC). It has been suspected by many that there is a fundamental association between the principles that govern immunological self-tolerance and experimentally observed Ir gene control of responses to exogenous antigens. Does MHC-linked unresponsiveness to a particular antigen result from cross-reactivity between the exogenous antigen in question and self at some critical level(s)? We have approached this question through the study of in vitro antibody responses to variants of insulin. The amino acid sequence of insulin is highly conserved and murine antibody responses to heterologous insulins are controlled by H-2 linked Ir genes. We have previously demonstrated that although pork insulin fails to stimulate antibody responses in nonresponder C57BL/10 mice, it does prime helper T (Th) cells that support secondary antibody responses to pork insulin. However, dominant, pork insulin-primed suppressor T (Ts) cells normally mask this helper T cell activity. Thus, nonresponsiveness to pork insulin is controlled by highly specific suppressor T cells that prevent the expression of function, but not the priming of helper T cells. Here, we extend these studies to demonstrate that T cells from nonresponder C57BL/10 mice immunized with pork insulin have primed helper T cells and dominant suppressor T cells that cross-react with autologous insulin. Furthermore, functinally analogous, autoreactive helper, and suppressor T cells are observed after immunization with rat insulin (the amino acid sequence of which is identical to mouse insulin). These data provide evidence that tolerance to autologous insulin can be medicated by the activation of dominant suppressor T cells and that the MHC-linked unresponsiveness to pork insulin, as observed in C57BL/10 mice, results from immunological cross-reactivity between pork and mouse insulins.