Background: C-X-C chemokine receptor type 4 (CXCR4) helps to retain the hematopoietic stem cells (HSC) in the bone marrow (BM). CXCR4 binds to its ligand CXCL12/SDF1 which is constitutively expressed in the BM thereby inhibiting the mobilization of CXCR4 expressing immune progenitor cells. Moreover, increased numbers of effector cells in the tumor microenvironment (TME) are directly correlated to enhanced immunotherapeutic efficacy. Therefore, we hypothesized that CXCR4 antagonism will result in movement of immune progenitor cells from BM to periphery leading to increased availability of T-cells in the periphery and better infiltration of effector cells into the TME. Methods: In TC-1 mouse tumor-model, we tested the effects of CXCR4 antagonist (BL-8040; 4 doses; 24 h apart; 20 mg/kg) on the tumor growth and mice survival in the presence of tumor-specific antigen priming using E7 peptide vaccine (3 doses, one week apart). Anti-tumor immune responses were determined in the tumor tissues obtained 3-4 days after the second vaccination using flow cytometry. Results: We found that BL-8040 given with specific antigenic stimulation results in significantly enhanced anti-tumor immune response, leading to a decrease in tumor growth (p≤0.001 at day 21) and prolonged mice survival. At day 35 after tumor implantation, 80% of mice survived in the BL-8040 + vaccine treatment group compared to 0% survival following BL-8040 or vaccine treatments. Interestingly, we also found that BL-8040 leads to a significant increase in the numbers of antigen-specific CD8⁺ T-cells in the TME. We also found that starting BL-8040 prior to or together with the priming of the mice did not affect the outcome, suggesting that the scheduling of BL-8040 does not affect the therapeutic outcomes. Conclusions: These results suggest that BL-8040 treatment enhances anti-tumor immune response by potentially increasing the immune progenitor cells in the periphery leading to a better immune response. These results also suggest that BL-8040, a CXCR4 antagonist, is a promising immune-modulatory agent with potent anti-tumor effects.