NK cells derived from single cells exhibit cytotoxic activity against K562 targets. Single CD34⁺/Lin⁻/CD38⁻ progenitors were cultured on AFT024 with IL-7, FL, KL, IL-2, and IL-3 (added only at culture initiation) for 42 days and wells with the highest proliferation were transferred to 24-well plates in media containing IL-2 alone for an additional 14 days. Cells were then counted and tested for cytotoxicity against chromium-labeled K562 targets. Results are from 13 NK cell populations derived from progeny of 13 single cells from two normal donors (data represent the mean ± SEM of the average of triplicate wells from each population).