LAK activity of purified cord blood and BM NK cells against YAC-1 target cells. NK cells were purified and cultured in IL-2 (500 U/mL). CD56⁺ cells comprised 86.7% ± 7.0% and 82.8% ± 9.0% of purified populations in cord blood (n = 5) and BM (n = 4), respectively. After 3 days, the cells were washed and their cytotoxicity against YAC-1 cells measured using the¹²⁵I-UdR release assay and at three E:T ratios. The points represent individual samples. The average spons/max ratio was 8.0%.