The purpose of this study was to analyze inhibitory effects of anti-4-1BB monoclonal antibody on melanoma metastasis The 4-1BB (CD137) T cell molecule is a member of the TNF receptor family and its activation by either 4-1BB ligand or antibody induces T cell activation and growth. In the present study, administration of anti-4-1BB mAb induced inhibition of melanoma metastasis. Agonistic anti-4-1BB mAb induced not only CD$8^+$4-1BBT cells but also CD$8^+$IFN-${gamma}$$^{+}$ T cell population. In the presence of anti-CD3 antibody, lymphocytes produced high levels of IFN-${gamma}$ and low levels of IL-4 in anti-4-1BB mAb treated group. Exposure of melanoma cells to IFN-${gamma}$ induced expression of MHC-I molecules. Thus, the increase in number of CD$8^+$T cells and enhanced MHC-I expression on B16F10 cells by augmented IFN-${gamma}$ production in response to anti-4-1BB mAb may result in suppression of tumor growth and metastasis.s.