The Non-obese diabetic (NOD) mouse with polyuria, glycosuria, ketonuria and weight loss with age is the animal model of human type I diabetes. To clarify which subset of T cells is responsible for the development of this type of diabetes, we analyzed the cell surface phenotypes of lymphocytes in the spleen and pancreas of NOD mice using immunofluorescent and immunohistochemical techniques. Our results indicate that T cells, especially Ly-1 and L3T4 positive helper T cells may play an essential role in the pathogenesis of autoimmunity in NOD mice. L3T4, the mouse homologue of human T cell antigen termed T 4 or Leu 3, is an antigen on mouse helper T cells that responds to antigens of the class II major histocompatibility complex (MHC). Female NOD mice were treated twice a week with injections of monoclonal antibody to L3T4. Administration of this antibody inhibited lymphocyte infiltration of the pancreatic islet (insulitis) and prevented hyperglycemia and glycosuria. These results lead to the conclusion that the autoimmune response in NOD mice is rstricted to class II MHC and is T cell dependent. Also, the manipulation of the OKT4 (Leu3)-positive subset with monoclonal antibody may provide effective therapy for type I diabetes in human.