CD56 ^dim NK cells lacking NKG2A and KIR are functionally immature. PBMCs were incubated overnight with IL-12 and IL-18 and then stained with anti-CD56, anti-CD3, a cocktail of NKG2A and KIR antibodies, and IFN-γ for (A) CD56^dim NKG2A⁻KIR⁻ cells or (B) CD56^dim NKG2A⁺ and/or KIR⁺ cells. Shown is a representative example of 12 experiments that all gave similar results. Sorted NKG2A⁻KIR⁻, NKG2A⁺KIR⁻, NKG2A⁻KIR⁺, and NKG2A⁺KIR⁺ populations were tested for cytolysis of K562 target cells in a 4-hour chromium release assay after (C) a 16-hour incubation with IL-2 (n = 6), to allow recovery of cytolytic machinery, or (D) after 14 days of maturation on EL08–1D2 and IL-15 (n = 4). After short-term culture with IL-2 the NKG2A⁻KIR⁻ population exhibited a marked lack of cytolytic activity compared with the other populations (all P < .05). After maturation in culture for 14 days this difference was no longer apparent.