56 Background: MDSCs are a phenotypically diverse population of bone-marrow-derived cells that play an important role in tumor progression based on their immunosuppressive and pro-angiogenic properties. Mobilization of MDSCs from the bone marrow into the blood stream is dependent on tumor derived factors. Therefore, it is likely that levels of circulating MDSCs in cancer patients are reflective of tumor activity. However, the clinical applicability of MDSCs as a predictive biomarker in cancer has been hindered by a lack of consensus on the surface markers that defines them. Although there have been several attempts at harmonizing the nomenclature and characterization standards of MDSCs, there is still a lot of confusion particularly in the detection of MDSCs in unfractionated peripheral blood. To address this issue we have devised a simple flow cytometric test that reliably detects in whole blood the MDSCs subtypes that have been consistently reported to have clinical relevance in cancer. Methods: Whole blood from cancer patients and normal volunteers was stained with Lin cocktail, HLADR, CD33, CD15, CD14, and CD11b antibodies. After staining, red blood cells were lysed, samples were washed and analyzed by flow cytometry. Results: This approach identifies monocytic MDSCs (Linlo/CD33+/HLADR-/CD14+/CD15-), granulocytic or polymorphonucleated polymorphonuclear MDSCs (Linlo/CD33+/HLADR-/CD14-/CD15+), and promyelocytic or immature MDSCs (Linlo/CD33+/HLADR-/CD14-/CD15-), as well as Linlo/HLADR-/CD33+/CD11b+ MDSCs. This latter subpopulation has been reported by us and others to correlate with disease stage and tumor burden in several solid malignancies. This assay also allows the accurate determination of absolute numbers of each MDSC population when white blood cell counts are taken into consideration. Conclusions: Our proposed simple and rapid flow cytometry-based assay allows the measurement of frequency and absolute numbers of circulating MDSC subpopulations in unfractionated blood, and could provide a reference for their further characterization as predictive biomarkers for solid malignancies.