Islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) has been identified as a novel CD8+ T cell-specific autoantigen in NOD mice. This study was undertaken to identify MHC class II-specific CD4+ T cell epitopes of IGRP. Peptides named P1, P2, P3, P4, P5, P6, and P7 were synthesized by aligning the IGRP protein amino acid sequence with peptide-binding motifs of the NOD MHC class II (IA g7) molecule. Peptides P1, P2, P3, and P7 were immunogenic and induced both spontaneous and primed responses. IGRP peptides P1-, P2-, P3-, and P7-induced responses were inhibited by the addition of anti-MHC class II (IA g7) Ab, confirming that the response is indeed IA g7 restricted. Experiments using purified CD4+ and CD8+ T cells from IGRP peptide-primed mice also showed a predominant CD4+ T cell response with no significant activation of CD8+ T cells. T cells from P1-, P3-, and P7-primed mice secreted both IFN-γ and IL-10 cytokines, whereas P2-primed cells secreted only IFN-γ. Peptides P3 and P7 prevented the development of spontaneous diabetes and delayed adoptive transfer of diabetes. Peptides P1 and P2 delayed the onset of diabetes in both these models. In summary, we have identified two IA g7-restricted CD4+ T cell epitopes of IGRP that can modulate and prevent the development of diabetes in NOD mice. These results provide the first evidence on the role of IGRP-specific, MHC class II-restricted CD4+ T cells in disease protection and may help in the development of novel therapies for type 1 diabetes.