Insulin dependent diabetes mellitus (IDDM) in the non-obese diabetic (NOD) mouse is the result of a cellular mediated autoimmune event that destroys pancreatic islet beta cells. This destruction is characterized by a progressive lymphocytic infiltration into the islets as well as circulating autoantibodies and T cells reactive with islet antigens. To gain a better understanding of the cells responsible for islet destruction we isolated lymphocytes from the islets of prediabetic NOD mice and conducted a comparative phenotypic analysis with the analogous subpopulations of lymphocytes isolated from peripheral blood and lymph node (LN) of the same mice. CD3+ cells were analysed for T cell receptor (TcR); cell bearing gamma delta TcR were consistently observed at a higher frequency in the infiltrating T cells than in the periphery. Lymphocytes were also characterized for the expression of CD4 and CD8 T cell markers and, within each population, for the expression of activation markers (CD25, CD69) and adhesion markers (CD51, CD54, CD11b, CD49e, L-selectin). Significantly increased levels of CD4+CD8+ double-positive and CD4-CD8- double-negative T cell populations were observed in the infiltrating lymphocytes as compared with peripheral lymphocytes. In addition, within both CD4 and CD8 subpopulations isolated from islet infiltrates, CD11b+ and CD49e+ cells were increased with respect to the same subset of cells isolated from the periphery. In contrast, the level of cells that expressed L-selectin was significantly higher in the periphery for both CD4+ and CD8+ cells than for infiltrating cells. These data describe the phenotype of islet reactive T cells in the NOD mouse and identify possible targets for therapeutic intervention.