There were few natural killer (NK) cells in the liver in very young mice at the age of 1-2 weeks. This was because the cell yield from the liver of young mice was low. The percentage of NK cells in the liver of young mice, however, was almost comparable with that in the liver of adult mice. Lymphocytes were isolated from the liver and spleen of C57BL/6 (B6) mice, and NK cytotoxicity and phenotype were herein examined in this study. NK cytotoxicity was extremely high in the liver of very young mice. This phenomenon was seen in the liver of various normal mouse strains. In contrast, the appearance of high cytotoxicity was not seen in NK cells of the spleen, irrespective of mouse strains. The quality of NK cells in the liver of young mice was different from that in adult mice. NK cells in the liver of young mice were mainly CD69(+)Mac-1(-) Fas ligand(+), whereas those in the liver of adult mice were CD69(-)Mac-1(+) Fas ligand(-). These results revealed that the quality of hepatic NK cells changes in the process of ageing. Namely, liver NK cells in very young mice temporarily show the highest NK cytotoxicity and a unique activated phenotype. Physiological meaning of the present phenomenon was discussed.