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We investigated the individual CD8+ populations with natural killer (NK) cell markers (NK-type T cell); CD56 single positive (CD56)-T cells, CD56/CD57 double positive (DP)-T cells and CD57 single positive (CD57)-T cells in the peripheral blood. All NK-type T-cell populations expressed CD122 and intermediate levels of T-cell receptor (TCR; regular CD8+ T cells are CD122- and express high levels of TCR). The number of both DP-T cells and CD57-T cells, but not CD56-T cells, gradually increased with age. All NK-type T-cell populations produced larger amounts of interferon-gamma than did regular CD8+ T cells after stimulation with interleukin (IL)-2, IL-12 and IL-15. However, CD56-T cells and CD57-T cells but not DP-T cells showed a potent antitumour cytotoxity to NK-sensitive K562 cells, whereas only CD56-T cells showed a potent cytotoxity to NK-resistant Raji cells. Furthermore, although NK-type T cells produced large amounts of soluble Fas-ligands, their cytotoxic activities appeared to be mediated by the perforin/granzyme pathway. The oligoclonal or pauciclonal expansions of certain VbetaT cells were found in each NK-type T-cell population. The non-variant CDR3 region(s) for the TCRbeta chain(s) showed CD57-T cells and CD56-T cells to be derived from distinct origins, while the DP-T cell population consisted of a mixture of the clones seen in both CD56-T cells and CD57-T cells. Our results suggest that CD57-T cells and CD56-T cells are functionally and ontogenically different populations while DP-T cells appear to originate from both CD56-T cells and CD57-T cells.