I nvariant NKT (iNKT) cells are potent activators of DCs, NK cells, and T cells, and their antitumor activity has been well demonstrated. A single injection of the high-affinity CD1d ligand α-galactosylceramide (αGalCer) leads to short-lived iNKT cell activation followed, however, by long-term anergy, limiting its therapeutic use. In contrast, we demonstrated here that when αGalCer was loaded on a recombinant soluble CD1d molecule (αGalCer/sCD1d), repeated injections led to sustained iNKT and NK cell activation associated with IFN-γ secretion as well as DC maturation in mice. Most importantly, when αGalCer/sCD1d was fused to a HER2-specific scFv antibody fragment, potent inhibition of experimental lung metastasis and established s.c. tumors was obtained when systemic treatment was started 2–7 days after the injection of HER2-expressing B16 melanoma cells. In contrast, administration of free αGalCer at this time had no effect. The antitumor activity of the CD1d–anti-HER2 fusion protein was associated with HER2-specific tumor localization and accumulation of iNKT, NK, and T cells at the tumor site. Targeting iNKT cells to the tumor site thus may activate a combined innate and adaptive immune response that may prove to be effective in cancer immunotherapy.