The triploid J strain Xenopus laevis (MHC haplotype, j/j/j) were thymectomized as early larvae, and in the adult stage each animal was given a pair of thymuses or lymphocytes from semiallogeneic diploid donor frogs (j/k). The alloreactivity of host frogs was restored to third-party donors in terms of skin graft rejection and mixed leukocyte reaction, but there was specific unresponsiveness against the k haplotype. Grafting of heavily irradiated (10,000 rads) thymuses also restored host reactivity with induction of tolerance to the k haplotype. In the latter frogs, all thymic and splenic lymphocytes were of host origin. Injection of splenocytes from the restored frogs into secondary thymectomized J frogs not only restored immunocompetence but also transferred specific tolerance to the k haplotype. Injection of lymphocytes from the restored frogs to normal frogs failed to transfer specific tolerance by both in vivo and in vitro immune tests. The results suggest that a selective deletion of the T cell population reactive to the k haplotype was maintained in the T-cell-restored frogs.