Natural killer (NK) The aim of the study was to analyze the frequency and activity of NK cells from the peripheral blood of healthy children aged 6-17 years (n = 18). The tests were performed with the use of flow cytometer EPICS XL (Beckman Coulter). We have shown that the cytotoxic activity of NK cells differed significantly from one another. The study proves that the overall cytotoxic activity of NK cells depends on the number of peripheral blood cells (p = 0.01) in a healthy children population.