During the first trimester of pregnancy NK cells represent >50% of the lymphoid cells present in the human decidua where they reside in close contact with trophoblast cells. Because in decidual tissues NK cell activation and function may be induced by this interaction, we analyzed the cellular ligands recognized by activating NK receptors expressed on trophoblast cells. We show that these cells primarily express the NKp44 and DNAM-1 ligands and that interaction between these ligands and their corresponding receptors results in NK cell triggering. While activated peripheral blood NK (pNK) cells lysed the trophoblast cell lines JAR and JEG3, decidual NK (dNK) cells did not. On the other hand, they released VEGF, SDF-1, IP10 and large amounts of IL-8. Interaction with K562 target cells was exploited to induce optimal NK cell triggering, allowing a parallel, quantitative assessment of both cytolytic activity and cytokine production elicited by dNK cells. While dNK cells were unable to kill K562 even at high effector:target (E:T) ratios, they released large amounts of IL-8 also at low E:T ratios, a scenario compatible with dNK trophoblast cells interaction occurring within decidual tissues.