Lining the luminal surface of prosthetic small diameter bypasses with endothelial cells (EC) will lower its thrombogenicity. Unfortunately, human EC are scarce. Mesothelial cells (MC) may be a valuable alternative for EC, since they are abundantly available and have antithrombotic and fibrinolytic properties. An important anticoagulant function of EC is due to thrombomodulin (TM) on the surface. The presence of TM on omentally derived human MC is not known but would increase the chance of successful use of MC for cell seeding procedures. The expression and localization of TM on human MC was studied using monoclonal antibodies. TM activity on cultured MC, and the influence of cytokines, was measured by the generation of activated protein C (APC), and was compared to EC. TM is expressed on the surface of MC as well as intracellularly, both in vivo and in vitro. The TM-dependent generation of APC was significantly higher on cultured MC than on cultured EC (817 +/- 141 PM v 262 +/- 38 PM; P < 0.001); their reaction to cytokines was almost identical. Seeding the MC onto vascular prostheses did not change the TM activity. Thrombomodulin is present and highly active on cultured and seeded MC. This may have major implications for MC as a source for cell seeding on vascular prostheses.