The effect of IL-12 concentration on induction of apoptosis of NK cells. Purified CD3⁻CD56⁺ human NK cells were cultured for 3 days in the presence of 15 ng/mL IL-15 and varying concentrations of IL-12. The percent of NK cells undergoing apoptosis was determined by PI-stained nuclei using flow cytometry. Results are representative of three experiments.