OBJECTIVE To determine the quantity and quality of liver NK cells from young and adult mice and compare their characteristics. METHODS C57BL/6 mice were used at 2 weeks (young) and 8 weeks (adult) of age. The percentage and absolute number of NK cells in the liver and spleen were analyzed. The cytotoxicity of NK cells in the liver and spleen against various targets were detected by a 4 h 51Cr-release method. FACScan was used to analyze the expression of CD69, Mac-1, Ly49C/I and CD94 on the NK cells. Perforin mRNA levels were analyzed by the reverse transcription polymerase chain reaction (RT-PCR). RESUTLS The percentages of NK cells in the liver of young and adult mice were similar (11.9%±1.7% vs. 9.9%±1.6%, P>0.05), but the absolute number per liver weight was higher in the young animals (11.6±2.5x105/g vs. 3.4±0.8x105/g, P<0.05). The level of NK cytotoxicity was extremely high in the liver of young compared to adult mice (71.0%±5.5% vs. 23.8%±4.4%, P<0.05), but this difference was not observed in the spleen. Phenotypes of the liver NK cells from young and adult mice were completely different from each other. The liver NK cells from young mice were CD69high Mac-1lowLy49C/ Ilow, whereas NK cells from older mice displayed inverse antigen levels (CD-69low Mac-1high Ly49C/Ihigh). The expression levels of other NK cell-related markers were similar in both groups.The perforin mRNA level in the liver lymphocytes from young mice was consistently greater compared to adult mice. CONCLUSION From 2 to 8 weeks C57BL/6 mice liver NK cells undergo age-associated changes. At 2 weeks of age the liver NK cells showed a high level of NK cytotoxicity and a unique phenotype which was not apparent at 8 weeks of age. Methods C57BL / 6 mice were used at 2 weeks (young) and 8 weeks (adult) of age. The percentage and absolute number of NK The cells in the liver and spleen were analyzed. The cytotoxicity of NK cells in the liver and spleen against various targets were detected by a 4 h 51 Cr-release method. FACScan was used to analyze the expression of CD69, Mac-1, Ly49C / I and CD94 on the NK cells. Perforin mRNA levels were analyzed by the reverse transcription polymerase chain reaction (RT-PCR). RESUTLS The percentages of NK cells in the liver of young and adult mice were similar (11.9% ± 1.7% vs. 9.9 % ± 1.6%, P> 0.05), but the absolute number per liver weight was higher in the young animals (11.6 ± 2.5 × 105 / g vs. 3.4 ± 0.8 × 105 / g, P < extremely high in the liver of young compared to adult mice (71.0% ± 5.5% vs. 23.8% ± 4.4%, P <0.05), but this difference was not observed in the spleen. Phenotypes of the liver NK cells from young and adult mice were completely different from each other. The liver NK cells from young mice were CD69high Mac-1lowLy49C / Ilow, (CD-69low Mac-1high Ly49C / Ihigh). The expression levels of other NK cell-related markers were similar in both groups. The perforin mRNA level in the liver lymphocytes from young mice was consistently greater than adult mice. CONCLUSION From 2 At 2 weeks of age the liver NK cells showed a high level of NK cytotoxicity and a unique phenotype which was not apparent at 8 weeks of age.