Manipulation and possible mechanism of pathogenesis in NOD mice were studied from the immunological standpoint. Disturbance of lymphocyte activity, particularly that of T-lymphocyte activity, greatly influenced the development of overt diabetes. Throughout the test period when the mice were between 24 and 30 weeks old, spontaneous diabetes in female mice was markedly suppressed by treatment with anti-thymocyte serum or with an anti-Thy 1,2 monoclonal antibody. Also, the appearance of diabetes was promoted by cyclophosphamide treatment in animals of both sexes. Evidence for the role of T-lymphocytes was provided by the experiments using the T-lymphocyte-depleted mice. Neither insulitis nor overt diabetes was observed in the athymic nude congenic mice with the genetic background of NOD even if they were given cyclophosphamide. Insulitis was induced in the athymic nude recipients by the transfer of thymocytes or of splenocytes from euthymic littermates, although an overt diabetes was not produced by cell transfer alone. An overt diabetes was induced in the congenitally athymic nude mice by a single transfer of T-lymphocytes and subsequent treatment with cyclophosphamide. From these results it can be concluded that T-lymphocytes play an obligatory role in the pathogenesis of diabetes in NOD mice.