NK cell progeny resulting from the three-step switch culture exhibited characteristic cytotoxicity against tumor targets. CD34⁺/Lin⁻/DR⁻ cells were plated in IL-3 and MIP-1α SNC culture for 14 days. Resultant bulk cultured cells (left panel, n = 4) or sorted CD34⁺/CD33⁻ cells (right panel, n = 6) were switched to conditions favoring NK cell development. NK cell progenitor commitment was induced by IL-2, IL-7, SCF, and stromal ligands. After a final NK cell expansion in IL-2–containing NK medium, cytotoxicity was tested against K562 (•) and Raji (○). Lytic activity for bulk switched or CD34⁺/CD33⁻cells was similar to that observed from fresh CD34⁺/Lin⁻/DR⁻ cells plated in long-term NK cell cultures for 5 weeks.