The authors assessed the activity of NK cells on a flow cytometer using 5-carboxyfluorescein diacetate as fluorescent substrate. They examined 25 healthy subjects aged 23-52 years. In NK cellular activity the normal distribution applies and the value of NK cellular activity for the whole group as regards the ratio of effector cells to target cells was 25:1, 23.46 +/- 15.84 percentage. The authors did not detect any significant differences in the activity of NK cells in relation to age and sex. They revealed statistically significant correlations of NK activity and the number of CD16+ and CD57+ cells as well as CD57+ CD3- cells. The optimal ratio of effector and target cells for assessment of NK activity was 25:1. Assessment of NK activity on a flow cytometer can be considered an equivalent substitute of the classical method, using radiochromium.