CD137, a costimulatory factor of TNFR family, is expressed on activated T cells and freshly isolated mouse dendritic cells (DCs). To date, there are only limited data dealing with the expression and the effect of CD137 on human DCs. We report in this work that CD137 can coexpress with CD137L on immature peripheral blood-derived human DCs (9.77%). The mature DCs stimulated by LPS showed a much higher level of CD137 expression (36.06%). Ligation of CD137 on the surface of DCs with anti-CD137 monoclonal antibody (mAb)could enhance the direct anti-tumor effect mediated by human DCs in vitro. The agonistic anti-CD137 mAb was able to elevate by about 20% of the DC-mediated inhibition of tumor growth in five tumor cell lines. These results indicate that the appliance of anti-CD137 mAb might be used as a new strategy for tumor immunotherapy. Cellular & Molecular Immunology. 2004;1(1):71-76.