Aims/hypothesis. Major histocompatibility complex class II molecules present antigenic peptides to T-cells and have an important role in T-cell thymic education. The mechanism by which major histocompatibility complex alleles confer a high genetic risk for autoimmune diabetes is not known. One hypothesis is that during positive thymic selection, the peripheral T-cell repertoire is modelled by major histocompatibility complex-restricted presentation of self major histocompatibility complex molecule-derived peptides, some of which mimic tissue autoantigens. The sequence similarity between a known T-cell epitope of glutamic acid decarboxylase-65, 509:VPPSLRTLED and the non-obese diabetic mouse class II major histocompatibility complex molecule I-A^g7 86:VPTSLRRLEQ is consistent with this. Methods. We measured spontaneous proliferation of peripheral T-cells from non-obese diabetic mice and other, non-diabetes-prone strains, to the I-A^{g7 86–101} and glutamic acid decarboxylase-65^509–524 peptides, binding of these peptides to intact I-A^g7 and assessed the effect of tolerance induction on diabetes development, by injecting young non-obese diabetic mice with high doses of peptide. Results. T-cells from non-obese diabetic, but not other mice strains, spontaneously proliferate to the I-A^{g7 86–101} and glutamic acid decarboxylase-65^509–524 peptides, but not control peptides. Both test peptides bind I-A^g7. Tolerance induction prolongs diabetes-free survival in non-obese diabetic mice when either the I-A^{g7 86–101} or glutamic acid decarboxylase-65^509–524 peptide, but not control peptide, is used. Conclusion/interpretation. A peptide from the unique class II major histocompatibility complex, diabetes-susceptibility molecule, I-A^g7, presented by I-A^g7 is a target of T-cell responses in diabetes-prone non- obese diabetic mice and tolerance induction against the peptide offers appreciable protection against the development of diabetes. [Diabetologia (1999) 42: 560–565]