Increasing activating ligand density on target cells augments the frequency of cytokine- and chemokine-producing CD56^dim NK cells. Before mixing with NK cells, S2 cells expressing ligands for NK-cell receptors, as indicated, were preincubated with serial dilutions of anti–S2 cell serum. Resting NK cells were incubated with S2 cells for 6 hours at 37°C. 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 NK cells producing IFN-γ, TNF-α, MIP-1α, and MIP-1β, as indicated, was determined by flow cytometry. Values represent mean ± SD of 3 different donors.