A new model has been developed to address the question of whether T cells that traverse an allogeneic thymus during early and late life become restricted to interact, in vivo, with other leukocytes and target cells that display the major histocompatibility complex (MHC) antigens of the thymus haplotype. Chimeras were made microsur- gically with pairs of 24-h-old Xenopus embryos such that the anterior region of an embryonic chimera contained the thymus anlagen and was of one MHC genotype, whereas the posterior region contained the anlagen of all hemopoietic cells and was of another genotype. Assays to determine the MHC haplotype restriction specificity of T cells in chimeras that had been reared through metamorphosis involved: specific antibody responses (IgM and IgG) to dinitrophenylated keyhole limpet hemocyanin; rejection of minor H locus disparate skin grafts that expressed the MHC antigens of either the thymus donor or the lymphocyte donor; and mixed leukocyte culture. MHC-mismatched chimeras displayed split tolerance since they accepted skin grafts of the thymus haplotype but had lymphocytes that proliferated in response to MHC antigens of the thymus donor strain as well as to MHC antigens of third-party donors. IgM responses of MHC-matched and MHC-mismatched chimeras and of nonchimeric controls did not differ. However, the IgG responses of MHC-mismatched thymus/ lymphocyte chimeras peaked later than those of MHC-matched chimeras and normal controls. Data from skin grafting protocols were consistent with the proposition that there may be in vivo selection of T cells reactive to minor H antigens presented in association with the MHC antigens of the thymus rather than the MHC antigens of the lymphocytes themselves. These data suggest that although it is not absolute, there is thymic selection of the T cell repertoire in Xenopus.