Phogrin (IA-2β), a major autoantigen in type 1 diabetes in man is recognized by peripheral T cells in the nonobese diabetic (NOD) mouse. CD4⁺ T-cell clones derived from immunized NOD animals elicit islet destruction in a disease transfer model. Spontaneous proliferative responses to the protein and derived peptide epitopes were detected in peripheral lymph node cells (LNC) of unprimed NOD mice but not BALB/c controls as early as 4 weeks of age at a time point when insulitis in NOD animals is minimal. Responses to irradiated NOD islet cells but not irradiated NOD spleen cells were observed for both male and female NOD animals. Insulin, phogrin and phogrin-peptide 7 (aa 755–777) but not phogrin-peptide 2 (aa 640–659) or tetanus toxin peptide were recognized as antigens. Islet cell-reactive and phogrin peptide 7-specific CD4⁺ T-cell lines were generated from splenocytes of unprimed 4-week-old NOD females and shown to secrete Th1-type cytokines. The results show that the phogrin molecule is targeted early in the course of disease in NOD animals at a time when circulating autoantibodies are absent and insulitis is minimal.