Bioengineered Listeria monocytogenes (Lm), which secrete antigens fused to a fragment of listeriolysin O (LLO) have been shown to be effective cancer immunotherapeutics in several mouse models of cancer, and are currently being evaluated in Phase 2 clinical trials for HPV-associated dysplasia and malignancies. Treatment with Lm-LLO immunotherapies results in a marked increase in tumor infiltrating lymphocytes, in particular CD8+T cells. The antigen-specific CD8+T cells have been observed to kill tumor cells in vitro, showing the functionality of the Lm-stimulated T cells. Lm-LLO immunotherapies also have an impact on suppressor cells in the tumor microenvironment. In various tumor models, treatment results in a decrease in the ratio of regulatory T cells (Tregs) to effector T cells in the tumors. Here, we report a corresponding decrease in the ratio, and number, of monocyte-derived suppressor cells (MDSCs) after Lm-LLO immunotherapy. These change are only seen in the tumor microenvironment. Further, both Tregs and MDSCs isolated from the tumor have a decreased ability to suppress the division of T cells after Lm-LLO immunotherapy. However, there is no change in functional ability of splenic Tregs or MDSCs. Thus, the efficacy of Lm-based immunotherapies on tumors is likely due to a combined increase in antigen-specific effector cells and a tumor specific decrease in the numbers and function of suppressor cells in the tumor microenvironment.