Costimulation of NK cells by exogenous cytokines enhances cytokine production by CD56^dim NK cells upon interaction with K562 cells. Resting NK cells were incubated alone or stimulated with cytokines IL-12 and IL-18 (10 ng/mL and 100 ng/mL, respectively) for 6 hours at 37°C with or without K562 cells. After stimulation, the cells were surface stained with fluorochrome-conjugated anti-CD56 mAb, fixed, permeabilized, and stained intracellularly with fluorochrome-conjugated mAbs to cytokines and chemokines. The percentage of CD56^dim or CD56^bright NK cells producing IFN-γ, TNF-α, MIP-1α, and MIP-1β, as indicated, was determined by flow cytometry. Values represent mean ± SD of 7 different donors. For clarity, selected statistical analyses are indicated. *P < .05, **P < .01, ***P < .001.