Autoantibodies to the islet-specific Zn transporter ZnT8 (Slc30a8), as well as CD4 T cells, have been identified in patients with type 1 diabetes. Here we examined for CD4 T-cell reactivity to ZnT8 epitopes in the NOD mouse. Immunization with a cytoplasmic domain of the protein or with peptides predicted to bind to I-Ag7 resulted in a CD4 T-cell response, indicating a lack of deletional tolerance. However, presentation by intraislet antigen-presenting cells (APC) to the T cells was not detectable in prediabetic mice. Presentation by islet APC was found only in islets of mice with active diabetes. In accordance, a culture assay indicated the weak transfer of ZnT8 reactivity from insulinomas or primary β-cells to APC for presentation to T cells. A T cell directed to one peptide (345–359) resulted in the transfer of diabetes, but only in conditions in which the recipient NOD mice or NOD.Rag1−/− mice were subjected to light irradiation. In late diabetic NOD mice, CD4 T cells were found as well as a weak antibody response. We conclude that in NOD mice, ZnT8 is a minor diabetogenic antigen that can participate in diabetes in conditions in which the islet is first made receptive to immunological insults.