The possible therapeutic benefits of B-cell depletion in combating tumoral immune escape have been debated. In support of this concept, metastasis of highly aggressive 4T1 breast cancer cells in mice can be abrogated by inactivation of tumor-evoked regulatory B cells (tBreg). Here, we report the unexpected finding that B-cell depletion by CD20 antibody will greatly enhance cancer progression and metastasis. Both murine and human tBregs express low levels of CD20 and, as such, anti-CD20 mostly enriches for these cells. In the 4T1 model of murine breast cancer, this effect of enriching for tBregs suggests that B-cell depletion by anti-CD20 may not be beneficial at all in some cancers. In contrast, we show that in vivo–targeted stimulation of B cells with CXCL13-coupled CpG oligonucleotides (CpG-ODN) can block cancer metastasis by inhibiting CD20^Low tBregs. Mechanistic investigations suggested that CpG-ODN upregulates low surface levels of 4-1BBL on tBregs to elicit granzyme B–expressing cytolytic CD8⁺ T cells, offering some explanative power for the effect. These findings underscore the immunotherapeutic importance of tBreg inactivation as a strategy to enhance cancer therapy by targeting both the regulatory and activating arms of the immune system in vivo. Cancer Res; 73(7); 2127–38. ©2013 AACR.