Restoration of T cell function of athymic BALB/c nu/nu mice was investigated after transplantation of xenogeneic thymic rudiments from 15-d-old embryonic rats into kidney subcapsule. The rudiments developed well and formed a proper thymus structure composed of donor epithelia and host lymphocytes. Examination of antibody responses to SRBC revealed that approximately half the normal number of indirect PFCs were observed. Skin grafts from syngeneic BALB/c mice and thymic donor rat strains were accepted, whereas those from allogeneic mice and the rats of other than donor strains were vigorously rejected. Thymus-grafted nude mice under a conventional environment survived without any evident infectious diseases. Histological and immunofluorescence studies, however, showed a high incidence of multiple organ-localized autoimmune diseases in thyroid, salivary gland, stomach, adrenal, prostate, ovary, and testis in mice that produced the corresponding autoantibodies. These results together suggested that rat thymic grafts reconstituted T cell functions of nu/nu mice to a considerable degree, but that organ-localized autoimmune diseases developed, probably because certain auto-antigens of the recipients were recognized by the newly reconstituted host immunity.