677 Orthotopic composite tissue (limb) transplantation in rats is a unique model for vascularized bone marrow transplantation. In this model, bone marrow cells and bone marrow stroma are transplanted by surgical means, thus creating immediate bone marrow "space" and engraftments. Although this composite/vascularized bone marow transplant model has been recognized as a better source for bone marrow reconstitution than to transplantation of cellular bone marrow cells (BMCs), it contains a skin component which is a difficult tissue for inducing transplantation tolerance. Methods: In order to study only aspects of vascularized bone marrow transplantation, we created a new isolated vascularized bone marrow transplant model without involving any integumentary or muscle tissues. The model involves dissecting all of the skin and muscle tissues from the femoral bone along with isolating key vessels. The model consisted of transplantating vascularized male femoral bone marrow grafts to the abdominal cavity of female rat recipients. The common iliac (or femoral) artery and vein were microsurgically anastomosed to the recipient abdominal aorta and inferior vena cava in an end-to-side fashion, respectively. Syngeneic Lewis (RT1l, n=20) and allogeneic BN (RT1n, n=10) donors were studied. To assess rejection, we utilized the polymerase chain reaction (PCR) and examined microchimerism of donor male BMCs in the perpheral blood of female recipient using rat Y chromosome specific primers. Results: All recipients were healthy and remained stable without major complications. Male BMCs were detected in all syngeneic recipient peripheral blood-but not in allogeneic blood-8 weeks after transplantation. Morphologically, syngeneic male Lewis bone marrow showed a near normal appearance, while allogeneic BN male bone marrow was clearly rejected and/or replaced by fibroblastic cells 4 weeks following transplantation. Microchimerism in the recipient peripheral blood was shown to be an important criterion for determining whether or not grafts were accepted or rejected in the acute phase. Conclusion: In summary, a new model of vascularized bone marrow transplantation was established. This consisted of the vascularized femoral bone marrow transplant (vFBMT). Further analyses regarding the ability of vFBMTs to induce systemic transplantation tolerance in adult rats will provide insights into not only various issues of immunology but also potential clinical application of vascularized bone marrow transplantation.