Mucosal delivery of soluble antigen induces systemic tolerance and has been applied to the prevention of autoimmune diseases. We have studied mucosal tolerance in autoimmune diabetes using the non-obese diabetic mouse model. Treatment of prediabetic mice with the pancreatic islet autoantigen insulin, by aerosol or intranasal delivery, reduces the incidence of diabetes and is associated with induction of CD8 (αα) γδ T cells, small numbers of which prevent adoptive transfer of diabetes. We examine the evidence for γδ T cells in mucosal tolerance and discuss possible mechanisms underlying the induction and action of insulin-induced CD8 γδ regulatory T cells. CD8 γδ cells constitute the most abundant subpopulation of intraepithelial lymphocytes (IELs), the major lymphoid cell compartment and first line of cellular immune defence in the mucosa. Induction of regulatory CD8 γδ T cells requires conformationally intact but not biologically active insulin. In contrast, intranasal (pro) insulin peptide, or oral insulin which is degraded in the gut, induces CD4 regulatory cells. Regulatory γδ T cells secrete interleukin-10 in pancreatic lymph nodes, which could account for the antidiabetic and bystander suppressor effect of naso-respiratory insulin. The physiological role of γδ IELs in maintaining peripheral self-tolerance deserves further study.