Background. In rodents, spleen allotransplantation (SpTx) induces tolerance. We investigated the induction of chimerism and donor-specific unresponsiveness following pig SpTx. Methods. Thirteen pigs underwent splenectomy (day 0); all received a blood transfusion. In 11/13 pigs, SpTx was performed across a MHC class I (n = 1) or full (n = 10) barrier; two control pigs received no SpTx. All pigs were monitored for chimerism, and anti-donor immune responses, including suppressor assays. Four pigs (two asplenic controls and two with SpTx) underwent delayed donor-matched kidney transplantation without immunosuppression. Results. Six of the 11 spleen grafts were lost from rejection (n = 5) or splenic vein thrombosis (n = 1), and five remained viable. All 11 SpTx recipients developed multilineage chimerism, but chimerism was rapidly lost if the graft failed. Two control pigs showed <6% blood chimerism for 4 and 11 days only. Pigs with functioning spleen grafts had multilineage chimerism in blood, thymus and bone marrow for at least 2-6 months, without graft-versus-host disease. These pigs developed in vitro donor-specific hyporesponsiveness and suppression. In 2 pigs tolerant to the spleen graft, donor MHC-matched kidney grafts survived for >4 and >7 months in the absence of exogenous immunosuppression; in two asplenic pigs, kidney grafts were rejected on days 4 and 15. Conclusions. Successful SpTx can result in hematopoietic cell engraftment and in vitro donor-specific unresponsiveness, enabling prolonged survival of subsequent donor-matched kidney grafts without immunosuppression. Copyright © 2005 by Lippincott Williams & Wilkins.