In some rodent strain combinations, allogeneic spleen transplantation induces tolerance spontaneously to itself and to other donor-specific organs. In other combinations, a state of tolerance has been achieved in the weakened immune system of the recipient. The data indicate that if a balance can be achieved between host-versus-graft and graft-versus-host responses, tolerance develops, possibly due to the development of suppressor/regulatory cells. There have been a number of unsuccessful studies in outbred large animals, but none in MHC-defined donor-recipient pairs, and none in which the protocol specifically aimed at inducing tolerance. Spleen transplantation has been performed in approximately 50 humans for a number of reasons, however no clear immunologic advantage has been reported. Graftversus-host disease (GVHD) was documented in at least 3 patients, and was lethal in one case, despite excision of the donor spleen. The advantages of tolerance over chronic immunosuppressive therapy are so great that a potentially tolerogenic approach such as spleen transplantation would seem worthy of further investigation in a suitable large animal model. Such a study is ongoing at our center.