Hemorrhage is the leading cause of preventable combat deaths, accounting for 90% of preventable mortality during Operation Iraqi Freedom and Enduring Freedom (OCT01-JUN11) [1]. Rapid fluid resuscitation saves lives and whole blood is associated with significant reductions in mortality compared to approved blood component therapy [2]. The US Military Operations in Iraq, Syria, and Afghanistan therefore re-introduced whole blood, often in the form of FDA-approved cold-stored whole blood (CSWB) as the ideal resuscitation fluid to treat the traumatically hemorrhaging warfighter [3]. The current Joint Trauma Systems Clinical Practice Guidelines (JTS CPG) support whole blood for Damage Control Resuscitation /Damage Control Surgery (DCR/DCS), second only to CSWB. However, CSWB is not always readily available in austere forward-deployed environments because CSWB requires refrigeration and frequent resupply due to limited shelf life. For these reasons, “Walking Blood Bank” (WBB) programs have been initiated. In WBB, pre-screened Low Titer O Whole Blood (LTOWB) can be quickly obtained from on-site donors. WBB does not require logistics sourcing, other than lightweight transfusion kits. The Ranger O Low Titer (ROLO) protocol, initiated in Afghanistan in MAR16, demonstrated the efficacy of WBB [4]. Crucial to WBB is ensuring that the donor is has type-O blood. Type-O blood is universal-donor blood, blood that can be received by the casualty without concern of transfusion reactions that can arise from blood type mis-match. As near peer conflict may be increasingly imminent, we anticipate prolonged casualty care (PCC) with dispersed small surgical units tending to the medical needs of Marines and members of other service branches. Logistics of resupply/storage of cold-stored LTOWB will be challenging at best and impossible at worst. WBB screening for type-specific blood can be cumbersome and take hours to complete, which is not ideal in a chaotic mass-casualty situation. In preparation of near-peer conflict, it would therefore be of great value to know the base rate of LTOWB in active-duty Marines. However, very little is known regarding the prevalence of potential LTOWB donors within active-duty Marines. What is therefore needed is an investigation to determine the base rate of LTOWB in active-duty Marines. Towards preserving the life of the traumatically wounded warfighter, the presently proposed study was specifically designed to fill this important gap.