We treated young NOD mice (2-week-old) with injections of anti-L3T4 mAb or anti-Lyt-2 mAb in order to elucidate the role of T cell subsets on the development of diabetes in NOD mice. Administration of these mAbs prevented insulitis and diabetes in the respective groups of NOD mice. These results were in contrast with our previous report (J. Japan Diab. Soc. 31: 275, 1988) that the administration of anti-L3T4 mAb to precritical NOD mice (12-week-old) prevented diabetes, while anti-Lyt-2 mAb failed to do so, and that severe insulitis similar to that in the control mice was observed in both groups. Based on these findings, together with our previous observations, we may conclude that cooperation between L3T4-positive T cells and Lyt-2 positive T cells is essential to the development of insulitis and that L3T4-positive T cells play a major role in the destruction of B cells in NOD mice.