In contrast to the well-established efficacy of preventive vaccines, the effectiveness of therapeutic vaccines remains limited. To develop effective vaccination regimens against cancer, we have analyzed the effect of effector and memory CD8+ T cells on the ability of dendritic cells to mediate the immunologic and antitumor effects of vaccination. We show that in contrast to effector CD8+ T cells that kill antigen-carrying dendritic cells, IFNgamma-producing memory CD8+ T cells act as "helper" cells, supporting the ability of dendritic cells to produce interleukin-12 (IL-12) p70. Promoting the interaction of tumor antigen-carrying dendritic cells with memory-type "heterologous" (tumor-irrelevant) CD8+ T cells strongly enhances the IL-12p70-dependent immunogenic and therapeutic effects of vaccination in the animals bearing established tumors. Our data show that the suppressive and helper functions of CD8+ T cells are differentially expressed at different phases of CD8+ T-cell responses. Selective performance of helper functions by memory (in contrast to effector) CD8+ T cells helps to explain the phenomenon of immune memory and facilitates the design of effective therapeutic vaccines against cancer and chronic infections.