A model of neonatal autoimmune disease has been described recently in which an epitope-specific autoantibody to murine zona pellucida 3 induces severe ovarian disease in neonatal, but not adult, mice (neonatal AOD). The autoantibody forms immune complex with endogenous ovarian zona pellucida 3, and a pathogenic CD4+ T cell response is triggered. The basis for the predominant neonatal susceptibility has not been clarified. In this study innate immunity, including neonatal NK cells, in neonatal AOD was investigated. Neonatal spleen contained readily detectable NK1.1+TCRVβ−, but not NK1.1+TCRVβ+, cells. Ab depletion of NK1.1+TCRVβ− cells inhibited neonatal AOD development. Moreover, in adoptive transfer of neonatal AOD, recipient disease was ameliorated when either donor or recipient NK cells were depleted. Thus, NK cells operate in both induction and effector phases of the disease. IFN-γ was produced by neonatal NK cells in vivo, and it may be important in neonatal AOD. Indeed, ovaries with neonatal AOD expressed high levels of IFN-γ and TNF-α which correlated with disease severity, and the disease was inhibited by IFN-γ or TNF-α Ab. Importantly, disease was enhanced by recombinant IFN-γ, and treatment of T cell donors with IFN-γ Ab also significantly reduced adoptive transfer of neonatal AOD. Finally, neonatal AOD was ameliorated in mice deficient in FcγRIII and was enhanced in FcγRIIB-deficient mice. We conclude that neonatal NK cells promote pathogenic T cell response at multiple stages during neonatal autoimmune disease pathogenesis. Also operative in neonatal AOD are other mediators of the innate system, including proinflammatory cytokines and FcγRIII signaling.