The goal of this study was to gather and compare kinematic response and injury data on both female and male whole-body Post-mortem Human Surrogates (PMHS) responses to Underbody Blast (UBB) loading. Midsized males (50th percentile, MM) have historically been most used in biomechanical testing and were the focus of the Warrior Injury Assessment Manikin (WIAMan) program, thus this population subgroup was selected to be the baseline for female comparison. Both small female (5th percentile, SF) and large female (75th percentile, LF) PMHS were included in the test series to attempt to discern whether differences between male and female responses were predominantly driven by sex or size. Eleven tests, using 20 whole-body PMHS, were conducted by the research team. Preparation of the rig and execution of the tests took place at the Aberdeen Proving Grounds (APG) in Aberdeen, MD. Two PMHS were used in each test. The Accelerative Loading Fixture (ALF) version 2, located at APG's Bear Point range was used for all male and female whole-body tests in this series. The ALF was an outdoor test rig that was driven by a buried explosive charge, to accelerate a platform holding two symmetrically mounted seats. The platform was designed as a large, rigid frame with a deformable center section that could be tuned to simulate the floor deformation of a vehicle during a UBB event. PMHS were restrained with a 5-point harness, common in military vehicle seats. Six-degree-of-freedom motion blocks were fixed to L3, the sacrum, and the left and right iliac wings. A three-degree-of freedom block was fixed to T12. Strain gages were placed on L4 and multiple locations on the pelvis. Accelerometers on the floor and seat of the ALF provided input data for each PMHS' feet and pelvis. Time histories and mean peak responses in z-axis acceleration were similar among the three PMHS groups in this body region. Injury outcomes were different and seemed to be influenced by both sex and size contributions. Small females incurred pelvis injuries in absence of lumbar injures. Midsized males had lumbar vertebral body fractures without pelvis injuries. And large females with injuries had both pelvis and lumbar VB fractures. This study provides evidence supporting the need for female biomechanical testing to generate female response and injury thresholds. Without the inclusion of female PMHS, the differences in the injury patterns between the small female and midsized male groups would not have been recognized. Standard scaling methods assume equivalent injury patterns between the experimental and scaled data. In this study, small female damage occurred in a different anatomical structure than for the midsized males. This is an important discovery for the development of anthropomorphic test devices, injury criteria, and injury mitigating technologies. The clear separation of small female damage results, in combination with seat speeds, suggest that the small female pelvis injury threshold in UBB events lies between 4 - 5 m/s seat speed. No inference can be made about the small female lumbar threshold, other than it is likely at higher speeds and/or over longer duration. Male lumbar spine damage occurred in both the higher- and lower lower-rate tests, indicating the injury threshold would be below the seat pulses tested in these experiments. Large females exhibited injury patterns that reflected both the small female and midsized male groups - with damaged PMHS having fractures in both pelvis and lumbar, and in both higher- and lower- rate tests. The difference in damage patterns between the sex and size groups should be considered in the development of injury mitigation strategies to protect across the full population.