BACKGROUND: Allogeneic mesenchymal stem cells (MSCs) show great potential for the treatment of military and civilian trauma, based on their reduced immunogenicity and ability to modulate inflammation and immune function in the recipient. Although generally considered to be safe, MSCs express tissue factor (TF), a potent activator of coagulation. In the current study, we evaluated multiple MSC populations for tissue factor expression and pro-coagulant activity in order to characterize safety considerations for systemic use of MSCs in trauma patients who may have altered coagulation homeostasis. METHODS: Multiple MSC populations derived from either human adipose tissue or bone marrow were expanded in the recommended stem cell media. Stem cell identity was confirmed using a well-characterized panel of positive and negative markers. Tissue factor expression on the cell surface was evaluated by flow cytometry with anti-CD142 antibody. Effects on blood coagulation were determined by thromboelastography (TEG) and calibrated automated thrombogram (CAT) assays using platelet poor plasma or whole blood. RESULTS: MSCs express tissue factor on their surfaces and are pro-coagulant in the presence of blood or plasma. The adipose-derived MSCs (Ad-MSC) evaluated were more pro-coagulant and expressed more tissue factor than bone marrow MSCs (BM-MSCs), which showed a greater variability in TF expression. BM-MSCs were identified that exhibited low pro-coagulant activity, whereas all Ad-MSCs examined exhibited high pro-coagulant activity. The percentage of cells in a given population expressing surface tissue factor correlates roughly with functional pro-coagulant activity. MSC tissue factor expression and pro-coagulant activity change over time in culture. CONCLUSIONS: All MSC populations are not equivalent; care should be taken to select cells for clinical use that minimize potential safety problems and maximize chance of patient benefit. Adipose-derived MSCs appear more consistently pro-coagulant than BM-MSCs, presenting a potential safety concern for systemic administration in coagulopathic patients. Donor variation exists between different cell populations, and culture handling conditions may also determine coagulation activity. Cells must be routinely monitored during preparation to ensure that they retain the desired characteristics prior to patient administration.