The nonobese diabetic mouse is a model of spontaneous type I diabetes mellitus. It is possible to induce diabetes in young, irradiated nonobese diabetic mice by using adoptive transfer of splenocytes or splenic T cells obtained from diabetic donors. This study demonstrates that the induction of diabetes in the adoptive transfer system is dependent on both the L3T4+ and Lyt-2+ subsets of T cells. Neither of these T cell subsets alone mediates the development of severe insulitis or diabetes when adoptively transferred to young, irradiated recipients. In addition, we show that both the L3T4+ and Lyt-2+ subsets must be obtained from diabetic donors in order to transfer diabetes; neither subset can be replaced with cells obtained from young, nondiabetic donors.