Mast cells (MCs) that are well known for their important effector function in IgE-associated immune responses play a key role in innate immune defenses. In this study, we investigate the interaction between MCs and NK cells in vitro and in vivo. We show that mouse bone marrow-derived cultured MCs activated with LPS, polyinosinic-polycytidylic acid, or CpG can stimulate NK cells to secrete increasing concentrations of IFN-gamma. MCs induce a 20-fold increase in IFN-gamma release from NK cells after LPS stimulation. This enhancement of IFN-gamma secretion is cell contact dependent and TNF-alpha independent. Furthermore, we show that this interaction is in part mediated by OX40 ligand on MCs. NK cell-mediated cytotoxicity was not affected by the presence of MCs. Intracellular IFN-gamma levels in splenic NK cells are significantly decreased after i.p. injection of LPS in mast cell-deficient (C57BL/6 Kit(wsh/wsh)) mice in comparison with wild-type mice. In conclusion, our data show for the first time a direct mast cell-dependent NK cell activation. This interaction might play an important role in innate immune defense, as it is dependent on the presence of stimulators relevant in innate immune responses.