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Skin allografts in rats were rejected, even if they were treated with a sub-optimal dose of FK506. However, skin allografts in rats which simultaneously received vascularized limb allografts were rejected more slowly. In this study, the authors examined the reasons why vascularized limb allografts have the ability to induce immunologic tolerance. For investigating which tissue components provide such tolerance, they compared the mean survival times (MST) of skin allografts on the backs of rats (used as an indicator of rejection), which were simultaneously engrafted with vascularized skin allografts, muscle allografts, joint allografts, and limb allografts. In the cases of limb allografts (MST=32.1 days) (these allografts contain bone-marrow cells), the skin allografts survived longer than vascularized skin allografts (MST=23.6 days) or muscle allografts (MST=22.6 days). For ascertaining whether the ability to induce immunologic tolerance of limb allografts was due to bone-marrow cells or not, the authors compared the MST of skin allografts in rats that were simultaneously engrafted with non-irradiated limb allografts, irradiated limb allografts, and bone-marrow cells. The allografted skin in the rats that received non-irradiated limbs (MST=32.1 days) survived longer than irradiated limb allografts (MST=20.3 days) and bone-marrow cells (MST=18.4 days). Limb allografts are, as it were, vascularized bone-marrow allografts. These results suggested that vascularized bone-marrow allografts can facilitate the induction of immunologic tolerance.