Nine nude mice were transplanted with cultured thymic fragments derived from syngeneic (three recipients) or allogeneic (six recipients) sources. All transplanted mice survived for periods of up to 8-10 mo thereafter, at which time they were sacrificed. Weight gain had been progressive and the animals were in excellent health. Four nontransplanted littermates housed in the same cages died at the age of 4 mo. In the nontransplanted mice, the usual deficits of T and B cells were observed. In transplanted mice, normalization of IgG1 and IgA levels as well as IgG antibodies to sheep erythrocytes and precipitating antibodies to rabbit serum occurred. Lymphocyte counts and Thy-1 bearing cells increased to approximately 50% of normal values. Proliferative responses to phytohemagglutinin and concanavalin A, mixed leukocyte reactivity, and cell-mediated lympholysis were variably restored from approximately 10-100% of normal. Attained responses were the same in recipients of syngeneic or allogeneic tissues and these, in turn, were equal or superior to responses measured in animals transplanted with whole noncultured thymuses. Skin grafts from third party donors were vigorously rejected, whereas those derived from second party (allogeneic thymus donor strain) may have been accepted or slowly rejected. Cultured thymic fragments, consisting primarily of epithelial elements, can effectively repair the thymic deficiency of nude mice. Experiments to date do not indicate that syngeneic tissues enjoy an advantage over allogeneic grafts in this restoration procedure.

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