In Xenopus, semi-xenogeneic JB skins render donor-specific tolerance to perimetamorphic JJ larvae, whereas the same grafts are never accepted by adults. To clarify the mechanisms underlying tolerance induction, we tried to find a method of inducing tolerance in adults. First, we reconstituted early-thymectomized (E-Txd) adults with larval or adult thymi. All JB skin grafts transplanted to E-Txd adults that had been reconstituted with larval thymi were rejected, while almost all of the E-Txd larvae that had been reconstituted with larval or adult thymi were rendered tolerant. Second, we transplanted tolerated JB (tol-JB) skin, i.e., JB skin that reportedly possessed a suppressive activity (Ono and Tochinai, 1995), to late-thymectomized adults and found that those adults were rendered tolerant. Third, when tol-JB skin and larval or adult thymi were simultaneously transplanted to E-Txd animals, many of the E-Txd adults were rendered tolerant. The overall results indicate that donor-specific tolerance can be induced in thymectomized JJ adults by cotransplanting either a larval or an adult thymus and a tol-JB skin graft.