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A class of immune suppressor cells termed myeloid-derived suppressor cells (MDSC) is known to be elevated in many solid tumor settings. MDSC are a heterogenous population of early myeloid cells that expand from the bone marrow during tumor progression and have the ability to inhibit both innate and adaptive immunity. Previous work from our laboratory and others indicated that patients with cancer exhibited reduced activation of signaling pathways when stimulated with Type I and II IFN as compared to normal donors. This reduced immune response could be a barrier to successful clinical outcomes, since ability to respond to IFN is known to be important for anti-tumor immunity and efficacy of immune-based therapies. We hypothesized that MDSC could inhibit an immune response to the growing tumor by reducing the ability of immune cells to respond to IFN stimulation. C26 tumor-bearing mice had significantly elevated levels of GR1 /CD11b MDSC as compared to control mice, and splenocyte