BALB/c mice were treated with fractionated high dose (3,400 rads) total lymphoid irradiation (TLI), and given semiallogeneic (BALB/c x C57BL/Ka) or allogeneic (C57BL/Ka) bone marrow and/or skin allografts. TLI alone prolonged the mean survival time (m.s.t.) of C57BL/Ka skin grafts to 49.1 days (control, 10.7 days). Shielding of the thymus during TLI produced only a slight increase in graft survival (m.s.t., 19 days). TLI combined with splenectomy was no more effective than TLI alone. Infusion of 10(7) semiallogeneic or allogeneic bone marrow cells after TLI produced stable chimeras in 7/8 and 8/15 recipients, respectively. Chimeras were specifically tolerant to donor tissues, since C57BL/Ka skin grafts were accepted for more than 250 days, but third-party (C3H/He) skin grafts were rejected rapidly. In addition, chimeric lymphocytes responded to C3H/He and C3H. Q but not to C57BL/Ka cells in the one-way mixed leukocyte reactions. BALB/c C57BL/Ka chimeras showed no clinical evidence of graft vs. host disease. These findings may have application of clinical organ transplantation, since (a) the recipient treatment (TLI) has already been shown to be safe in humans, (b) donors and recipients can be completely allogeneic, and (c) bone marrow and skin graft survival was permanent (greater than 250 days).

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