NK cells constitutively express monocyte-derived cytokine (monokine) receptors and secrete cytokines and chemokines following monokine stimulation, and are therefore a critical component of the innate immune response to infection. Here we compared the effects of three monokines (IL-18, IL-15, and IL-12) on human NK cell cytokine and chemokine production. IL-18, IL-15, or IL-12 alone did not stimulate significant cytokine or chemokine production in resting NK cells. The combination of IL-18 and IL-12 induced extremely high amounts of IFN-γ protein (225±52 ng/ml) and a 1393±643-fold increase in IFN-γ gene expression over those in resting NK cells. IL-15 and IL-12 induced less IFN-γ protein (24±10 ng/ml; p< 0.007) and only a 45±19-fold increase in IFN-γ gene expression over those in resting NK cells. The CD56 bright NK cell subset produced significantly more IFN-γ following IL-18 and IL-12 compared with CD56 dim NK cells (p< 0.008). However, the combination of IL-15 and IL-12 was significantly more potent than that of IL-18 and IL-12 for NK cell production of IL-10, macrophage inflammatory protein-1α, macrophage inflammatory protein-1β, and TNF-α at the protein and transcript levels. Granulocyte-macrophage CSF was optimally induced by IL-15 and IL-18. Resting CD56+ NK cells expressed IL-18R transcript that was up-regulated by IL-12 or IL-15. Our results show that distinct cytokine and chemokine patterns are induced in NK cells in response to different costimulatory signals from these three monokines. This suggests that NK cell cytokine production may be governed in part by the monokine milieu induced during the early proinflammatory response to infection and by the subset of NK cells present at the site of inflammation.