Intrathymic transplantation of syngeneic islets into adolescent NOD/Lt mice was performed to establish whether the thymus would serve as an immunoprivileged site for beta-cell engraftment, and whether this treatment would prevent the development of diabetes by eliciting tolerance to islet antigens. Intrathymic injection of cells from 200 NOD islets into 4-wk-old female NOD/Lt mice produced a significant reduction in the severity of insulitis at 24 wk of age. Furthermore, diabetes development was strongly suppressed (11% incidence) compared with controls (100% incidence). Both thymus histology and thymic insulin content revealed a rapid loss of the implanted beta-cells with < 1% remaining 1 wk posttransplantation. Despite the rapid loss of thymus-implanted islet cells, evidence for tolerance induction to islet cell antigens was obtained by adoptive transfer of splenic leukocytes from these mice into NOD-scid/scid recipients. After adoptive transfer of splenic leukocytes from 24-wk-old untreated prediabetic donors, 4 of 5 NOD-scid/scid recipients developed diabetes within 4 wk, and none of the recipients became diabetic after transfer of splenocytes from intrathymic islet-implanted donors. Intrathymic islet transplantation did not lead to reduction of sialitis in females with reduced severity of insulitis, indicating that the protective effect was tissue specific. This also was reflected in adoptive transfer experiments, because equal severity of sialitis was observed in NOD-scid/scid recipients of spleen cells from either islet transplanted or control NOD/Lt mice. In conclusion, the data suggest that intrathymic injection of islet cells prevents diabetes by stimulating immunological tolerance to beta-cells.