Background.Mesenchymal stem cells (MSC) are pluripotent progenitors for a variety of cell types, including fibroblasts and muscle cells. Their involvement in the tissue repair of allografts during the development of chronic rejection has been hypothesized, but not yet substantiated, by experimental evidence.Methods.Rat MSC were isolated from circulation using an aortic pouch allograft as a trapping device. The plasticity of these cells was examined in differentiation cultures. One of the resulting MSC lines was immortalized and transduced to express a markerlaczgene. Thelacz-labeled cells were then transferred to F344 rats bearing Lewis (LEW) cardiac allografts to measure their localization and contribution to graft tissue repair.Results.The MSC isolated from circulation exhibited multipotential for differentiation in culture, developing into various lineages including osteoblasts, lipocytes, chondrocytes, myotubes, and fibroblasts. Intravenous engraftment of thelacz-labeled cells into recipients of heart transplant resulted in migration of the &bgr;-gal+ cells into the lesions of chronic rejection in the cardiac grafts and homing of the cells to the bone marrow. The majority of &bgr;-gal+ cells present in the allografts exhibited fibroblast phenotypes, and a small number of the cells expressed desmin, indicative of myocyte differentiation.Conclusion.MSC vigorously migrated into the site of allograft rejection. This data suggests that they may be attracted to this site to actively participate in tissue repair during chronic rejection. In addition, given the robust migration, the inhibition of MSC differentiation toward fibroblast progeny and induction toward the myocyte lineage may serve as a new strategy for treatment of chronic rejection and allograft tissue repair.