Short-term culture with IL-12 increases the frequency of CD94:NKG2A⁺NK cells. Purified NK cells from different donors were cultured in medium alone or medium supplemented with IL-12 (10 ng/mL) (A) or IL-2 (250 U/mL) (B) for 24 hours. The NK cells were then stained with anti-CD94:NKG2A (mAb Z199) and analyzed by flow cytometry. Culture with IL-12 gave a median increase in CD94:NKG2A-expressing NK cells of about 12% (n = 14; P < .000 03), whereas culture with IL-2 had no significant effect (n = 11; P > .05). Purified NK cells were depleted of the CD94:NKG2A⁺ population. The depleted cell population was cultured for 24 hours in the absence of cytokine (control), with 250 U/mL IL-2, or with increasing doses of IL-12 (0.1 and 50 ng/mL) and then assayed for CD94:NKG2A expression (C).