The NOD mouse, which spontaneously develops insulitis and overt diabetes, is a model of autoimmune type I diabetes mellitus. To analyze of the roles of CD4+ and CD8+ T cells in the pathogenesis of this mouse, we have been doing a series of studies on the induction of insulitis and diabetes in NOD athymic nude mice by means of T cell transfer. To complement our previous study dealing with the induction of insulitis and cyclophosphamide-induced diabetes in recipients that had been reconstituted with each T cell subset derived from the nondiabetic mice, the present study was conducted to observe the transfer of spontaneous diabetes by injecting T cells harvested from diabetic mice. Any possible in vivo increase in the contaminating T cell subset was prevented by injecting the antibody homologous to it. Transfer of untreated or complement-treated splenic lymphocytes of diabetic mice containing both T cell subsets induced spontaneous diabetes 30 to 58 days after the cell transfer as well as insulitis, while spleen cells from nondiabetic mice rarely produced diabetes. On the other hand, transfer of either CD4+ cell-depleted or CD8+ cell-depleted splenic lymphocytes of diabetic mice did not cause diabetes at least up to 60 days after the cell transfer. Also, transfer of only the CD4+ T cell-depleted fraction did not cause insulitis. In contrast, transfer of only the CD8+ T cell-depleted fraction induced insulitis in all the recipients. However, insulitis was less potent in this group of mice than in the diabetic recipients given both subsets: only a low insulitis score was obtained and a number of beta-cells remained alive despite the insulitis in mice given the CD8+ T cell-depleted fraction, whereas islet damage was very severe and insulin-secreting beta-cells were no longer detected in the diabetic mice. Thus, the present results agree with the previous ones concerning transfer of diabetes, with the aid of cyclophosphamide treatment, by T cells of nondiabetic mouse origin. Consideration of our results together with earlier findings led to the conclusion that CD4+ T cells are primarily responsible for insulitis and that CD8+ T cells migrate into islets and are differentiated into mature killer cells against beta-cells with the aid of CD4+ T cells in both spontaneous and cyclophosphamide-induced diabetes.