Interleukin-12 (IL-12) and type 2 NO synthase (NOS2) are crucial for defense against bacterial and parasitic pathogens, but their relationship in innate immunity is unknown. in the absence of NOS2 activity, IL-12 was unable to prevent spreading of Leishmania parasites, did not stimulate natural killer (NK) cells for cytotoxicity or interferon-gamma (IFN-gamma) release, and failed to activate Tyk2 kinase and to tyrosine phosphorylate Stat4 (the central signal transducer of IL-12) in NK cells. Activation of Tyk2 in NK cells by IFN-alpha/beta also required NOS2. Thus, NOS2-derived NO is a prerequisite for cytokine signaling and function in innate immunity.