The importance of the immune response in cancer and other diseases (like diabetes mellitus, alzheimers, cystic fibrosis) is now known, and the manipulation of the immune system as a therapy to treat cancer is gaining attention. The immune system regulates tumorigenesis both negatively and positively. The myeloid-derived suppressor cells (MDSCs) are a population of immune cells that are increased during cancer, inflammation, and infection. These cells influence the immune response and effectively suppresses the anti-tumor T cell response. They serve as potential targets for therapeutic intervention to effectively use the immune system to inhibit tumorigenesis. To better understand how such intervention can be applied it is important to study these cell types. Using mouse ovarian tumors, we describe the isolation of MDSCs from solid tissue using gentle dissociation techniques. We further describe how MDSCs are isolated from such dissociated tissue based on the expression of cell surface markers with the help of flow cytometry. Additionally, we describe the procedure to perform an in vitro MDSC migration assay to determine the migration potential of these cells in response to soluble factors like cytokines and chemokines.