Abstract Thrombomodulin (TM) is a surface glycoprotein that forms a 1:1 complex with thrombin, thereby interacting to form the basis of a major physiologically relevant natural anticoagulant mechanism. Although initially described as a vascular endothelial cell receptor, TM has been reported to be present in several other cells, including megakaryocytes, platelets, monocytes, and several cultured cells. Other investigators have reported that neutrophils (PMN) may play a role in the hemostatic mechanism by supporting transformation of prothrombin to thrombin. To determine whether PMN might contribute further to the regulation of the coagulation system, we have evaluated these cells for the expression of TM. Large numbers of human leukocytes were isolated by standard techniques, and the PMN fraction was extracted and shown to be free of platelets and monocytes. Membrane preparations were affinity purified on an anti-TM-Affigel-10 matrix and the eluted material was examined by Western blotting, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and silver staining. The purified material was identical in apparent molecular weight to TM from human placenta and human umbilical vein endothelial cells (HUVEC). Using a sensitive and specific immunoassay, we estimated that there are a minimum of 5,220 +/- 1,658 molecules of TM per PMN, as compared with more than 50,000 in HUVEC. Northern analysis of RNA from PMN indicates that specific messenger RNA for TM, as identified by a single 3.8-kb band, is identical to that from HUVEC, and thereby confirms that PMN can also synthesize the receptor. Localization of TM in PMN was attempted by immunofluorescence, and the receptor was visualized only in permeabilized PMN, but was not seen on the surface of nonpermeabilized cells. Flow cytometry was also used, and could detect TM in 10% to 15% of nonpermeabilized PMN, whereas the antigen was present in greater than 80% of permeabilized cells. Biologic function of the PMN-derived TM, as tested by thrombin-dependent activation of protein C, was absent. Our results suggest that TM is synthesized by PMN, but under nonstimulated conditions, the protein is largely excluded from the membrane surface, and lacks the ability to promote activation of protein C by thrombin. TM from PMN may provide a further link between inflammation and thrombosis and may also be a significant source of plasma TM.