Cell therapy is a potentially powerful tool in the treatment of many grave disorders including leukemia, immune deficiencies, autoimmune diseases, and diabetes. However, finding matched donors is challenging and recipients may suffer from the severe complications of systemic immune suppression. Sertoli cells, when cotransplanted with both allo- and xenograft tissues, promote graft acceptance in the absence of systemic immunosuppression. How Sertoli cells do this is not, as yet, clearly defined. We have examined the ability of Sertoli cells to produce systemic immune tolerance. For this purpose, Sertoli cells were injected into an otherwise normal C57/BL6 mouse host via the lateral tail vein. No other immunosuppressive protocols were applied. Six to 8 weeks posttransplantation, blood was collected for analysis of cytokine levels. Tolerance to donor cells was determined by mixed lymphocytic culture, and production of T-cell-dependent antibody was determined by an in vitro anti-sheep red blood cell plaque-forming assay. Results showed a marked modulation of immune cytokines in the transplanted mouse host and donor-specific transplantation tolerance was achieved. Tolerant mouse lymphocytes maintained a competent humoral antibody response. Additionally, C57/BL6 mice transplanted with rat Sertoli cells tolerated rat skin grafts significantly longer than control non-Sertoli cell transplanted mice. We conclude that systemic administration of rat Sertoli cells across xenogenic barrier induces transplantation tolerance without altering systemic immune competence. These data suggest that Sertoli cells may be used as a novel and potentially powerful tool in cell transplantation therapy.