NKT cells promote antibody-induced arthritis, but the mechanism by which NKT cells are activated in this model remains unclear. It has been proposed that Fcgamma receptor (FcgammaR) contributes to NKT cell activation in antibody-induced arthritis. To address this issue, we explored the functions of FcgammaR on NKT cells in antibody-induced arthritis. RT-PCR and flow cytometric analysis demonstrated that NKT cells constitutively express surface FcgammaRIII but not FcgammaRI, -II, or -IV. FcgammaRIII engagement by aggregated IgG on NKT cells enhanced CD25 and CD69 expression, whereas FcgammaR(-/-) mouse NKT cells did not enhance activation. FcgammaRIII engagement on NKT cells enhanced the production of IL-4, IL-10, IL-13, and IFN-gamma, whereas FcgammaR-deficient NKT cells did not alter the production of these cytokines after aggregated IgG treatment. However, FcgammaR-deficient NKT cells were functionally intact in terms of TCR-induced activation. Moreover, adoptive transfer of FcgammaR-deficient NKT cells could not restore inflammation or TGF-beta production in the joint tissues of CD1d(-/-) mice, whereas adoptive transfer of wild-type NKT cells induced arthritis and reduced TGF-beta production in joint tissues. We conclude that FcgammaRIII engagement by IgG in joint tissues provides activating signals to NKT cells in antibody-induced arthritis.