Recent studies provided evidence that functional NK lymphocytes can be derived in vitro from immature cell precursors present in embryonic liver or postnatal thymus. In addition, by applying appropriate culture conditions a lymphoid cell population sharing NK and T-cell properties could be obtained from immature thymic cell precursors. Moreover, the molecular analysis of TCR/CD3 genes revealed that this cell population could represent a frozen stage of the T-cell lymphoid maturation. Altogether, these findings support the concept that a common precursor exists for T and NK cell lineages. In addition the availability of culture conditions which mimic the in-vivo microenvironment required for NK cell maturation may be important to investigate the role of self HLA in the selection of NK lymphocytes equipped with surface receptors specific for autologous HLA-class I alleles.