Trauma-related injuries are the fourth leading cause of death in the US. Elderly patients (>55 years) are more susceptible to trauma and often suffer worse outcomes after trauma than younger patients, but it is still unclear why elderly patients are often unable to recover fully. The murine polytrauma model of hemorrhagic shock and severe life threatening injury resembles the human response to trauma. Aged PT mice were found to produce less functional myeloid cells after trauma, evident in their decreased efficiency in clearing bacterial infections. The difference between the myelopoietic response of young and aged PT mice that gives rise to less functional myeloid cells could explain the disparity between the outcomes for elderly and young trauma patients. We expect that bone marrow progenitor cells isolated from elderly PT mice will give rise to fewer mature leukocytes than the same concentration of BM progenitor cells from young PT mice. Thus, we hypothesize that myeloid progenitor cells generated by emergency myelopoiesis in elderly PT mice have a decreased capacity for proliferation and that elderly PT mice have larger populations of myeloid-derived suppressor cells (MDSC).