The non-obese diabetic (NOD) mouse spontaneously develops a T cell-mediated autoimmune disease, sharing many features with human insulin-dependent diabetes mellitus (IDDM), leading to insulin-secreting beta cell destruction. The role of CD4+ T cells has been evidenced at two levels. First, CD4+ T cells from diabetic animals are required to transfer diabetes to non-diabetic recipients in conjunction with CD8+ effector T cells. Second, suppressive CD4+ T cells have been characterized in non-diabetic NOD mice. T cells with different functions can thus share the CD4+ phenotype. Since CD4+ T cells can be divided into at least two subgroups on the basis of CD45 isoform expression, we evaluated the distribution of CD4+ T cells expressing the CD45RA isoform on NOD mouse thymocytes and peripheral T cells. The percentage of CD45RA+ cells was dramatically increased among the most mature CD3bright thymocytes and among CD4+ T cells in lymph nodes of the NOD mouse as compared with control strains. This increase was related to the development of insulitis. Interestingly, the CD45RA isoform was expressed on most CD4+ T cells invading the islets. In vivo treatment with an anti-CD45RA mAb prevented the development of insulitis and spontaneous diabetes in female animals but not the transfer of diabetes by T cells collected from diabetic NOD donors. These results indicate that anti-CD45RA mAb is only effective if given before the full commitment of effector T cells to the destruction of islet beta cells. Thus CD4+CD45RA+ T cells play a key role in early activation steps of anti-islet immunity.