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Summary: The Xenopus early-thymectomy model system is used to investigate the extent to which the thymus controls T-cell development and to probe the evolution of natural killer (NK) ceils. Loss of T-cell function following thymectomy, together with the paucity of cells expressing monoclonal antibody-defined T-cell surface markers, and greatly reduced expression of T-cell receptor β transcripts in spleen, ever and intestine, indicate that T-cell development is minimal in the absence of the thymus. Our findings therefore mitigate against the idea that a substantial extrathymic pathway of T-cell development exists in early vertebrate evolution. Rather, they suggest that in this amphibian representative T cells are predominately thymus dependent. In vitro studies with control and thymectomized Xenopus splenocytes reveal that a non-T/non-B population and also two T-cell subsets all display natural cytotoxicity towards allogeneic thymus lymphoid tumour cells (which are deficient in MHC antigen expression). Since Xenopus thymectomized early in larval development are permanently deficient in T cells, they may provide a useful phylogenetic model for the study of NK cells.