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We reported previously that vascularized bone marrow transplantation (YBMT) in an orthotopic hind limb graft brings about complete repopulation of bone marrow cavities in lethally irradiated syngeneic recipients within 10 days. Intravenous infusion of an equivalent volume of bone marrow cell suspension was evidently less effective. The purpose of this study was to investigate the reconstitution of immunocompetent compartments of lethally irradiated syngeneic rats after VBMT. Lewis rat hind limbs were transplanted orthotopically into irradiated recipients. Ten days after irradiation and bone marrow transplantation, bone marrow, mesenteric lymph nodes and sera from rats were harvested. Responsiveness of mesenteric lymph node lymphocytes (MLNL) to mitogens and lymphocyte proliferation in the presence of sera and bone marrow cell (BMC)culture supernatants was measured. Our studies have shown that vascularized bone marrow transplantation brings about rapid replenishment of lymphoid organs of lethally irradiated syngeneic recipients. The repopulating subsets were fully responsive to mitogens. Sera from reconstituting rats had no evident effect on proliferation of mature lymphocytes. Intravenous infusion of BMC in suspension, in a number equivalent to that grafted in hind limb transplant, was less efficient in reconstituting lymphoid tissue.