Six CD4⁺ and three CD8⁺ islet-reactive T-cell clones were established from lymphocytes infiltrating the pancreatic islets of NOD mice. Two of six CD4⁺ T-cell clones responded to NOD islet cells only, not to spleen cells. The remaining four clones responded to both islet cells and spleen cells from NOD mice, but not to cells from other strains of mice, including SJL, C3H, C57BL/6, and DBA/2 mice. None of the CD4⁺ T-cell clones had a cytotoxic effect on the cultured islet cells. On the other hand, all of the CD8⁺ T-cell clones showed both a proliferative response and a cytotoxic effect on the islet cells, with the restriction of MHC class I H-2D^b. Electron microscopic studies revealed that islet-specific CD4⁺ T-cells attached closely to islet cells but did not destroy them. In contrast, CD8⁺ T-cell clones showed pseudopodialike protrusions into β-cells, but not α- or δ-cells, leading to selective destruction of β-cells. CD8⁺ CTLs could not be isolated from islets of NOD mice < 10 wk of age, even if the islets showed lymphocytic infiltration, whereas CD4⁺ T-cells could be isolated from islets of these younger NOD mice. On the basis of these observations, we concluded that CD4⁺ and CD8⁺ T-cells interact differently with β-cells at different stages in T-cell–mediated β-cell destruction. CD4⁺ T-cells may secrete cytokines, which in turn activate effector cell populations, whereas CD8⁺ T-cells may act as a final effector directly involved in β-cell destruction.