We analyzed the surface phenotypes of infiltrated cells in pancreases of nonobese diabetic (NOD) mice with monoclonal antibodies to mouse lymphocytes. Most of the infiltrated cells were Thy1⁺ and Ly1⁺ T-lymphocytes, and most of them were L3T4⁺ helper T-lymphocyte subsets. To elucidate the role of L3T4⁺ T-lymphocytes in the development of insulitis and diabetes in NOD mice, we treated the animals with injections of monoclonal anti-L3T4 antibody. Administration of this antibody prevented the insulitis and diabetes in NOD mice. These results lead to conclusions that the L3T4⁺ helper T-lymphocytes may play an essential role in the pathogenesis of type I diabetes and that the manipulation of the OKT4⁺ (Leu3⁺) T-lymphocyte subset, the human homology of L3T4, with monoclonal antibodies may provide effective therapy for human type I diabetes.