Since human endothelial cells synthesize Factor V but do not secrete it into the medium, we studied the effects of cell injury on the availability of Factor V at the surface of these cells. Human umbilical vein endothelial cells (HUVEC), grown to confluency and incubated with human 125I Factor Va, specifically bound 5000 to 7000 molecules per cell. In the absence of added Va, no antigen was detected on adherent HUVEC with either labeled anti-V(Va) monoclonal or polyclonal IgG. However, exogenous Va, not V, prebound to these cells allows binding of labeled 125I anti-V(Va). Immunodectectibility of bovine Factor V contributed by fetal calf serum in the concentration used in cultures is less than 0.1% of that detected in human plasma. HUVEC, suspended by scraping from dishes, specifically bound 4000 molecules/cell of 125/I monoclonal IgG against V(Va). Although undisturbed cells excluded trypan blue, dye uptake by many of the suspended HUVEC indicated cell injury. Quantitation of injury by 51Cr release after scraping followed by multiple passages through an 18 g needle showed that 51Cr release increased with number of manipulations up to 60% and was observed almost immediately after manipulation. We suggest that little Factor V(Va) is present on the surface of intact adherent HUVEC. However, mechanical injury to HUVEC released or exposed endogenous Factor V(Va), resulting in expression of V that might mediate Factor Xa binding as well as activation of protein C by thrombin. Thus, injured, but not intact, HUVEC could participate in both promoting and limiting blood coagulation.