Abstract: The activity of natural killer (NK) cell in 6 uremic patients and 8 healthy control subjects were measured with the method of ⁵¹Cr release. In control group, the activity of NK in female was 22.9±0.4% but was 36.6±1.9% in male which was higher than that in the female. In ure mia, the activity of NK was only 5.3±0.7% which may predispose the patients suffering from chronic renal failure to infections and cancers.