Anti-4-1BB-mediated anticancer effects were potentiated by depletion of CD4+ cells in B16F10 melanoma-bearing C57BL/6 mice. Anti-4-1BB induced the expansion and differentiation of polyclonal tumor-specific CD8+ T cells into IFN-gamma-producing CD11c+CD8+ T cells. The CD4+ cell depletion was responsible for facilitating immune cell infiltration into tumor tissues and removing some regulatory barriers such as T regulatory and indoleamine-2,3-dioxygenase (IDO)+ dendritic cells. Both monoclonal antibodies (mAb) contributed to the efficient induction of MHC class I molecules on the tumor cells in vivo. The effectors that mediated the anti-4-1BB effect were NKG2D+KLRG1+CD11c+CD8+ T cells that accumulated preferentially in the tumor tissues. Blocking NKG2D reduced the therapeutic effect by 20% to 26%, which may indicate that NKG2D contributes partially to tumor killing by the differentiated CD8+ T cells. Our results indicate that the combination of the two mAbs, agonistic anti-4-1BB and depleting anti-CD4, results in enhanced production of efficient tumor-killing CTLs, facilitation of their infiltration, and production of a susceptible tumor microenvironment.