We investigated the role of the endothelial cell (EC) receptor, thrombomodulin (TM), in angiogenesis by determining its contribution to EC microtubule formation, invasion and cell-cell adhesion. We first established a method to quantitate microtubule formation in vitro using stereology. Digital images of HUVECs grown in Matrigel™ were overlaid with a 4800-pixel grid. The mean number of points where one of the grid's vertices crossed a microtubule was determined and divided by the number of vertices per well (surface area covered by microtubules). Using this quantitative method, we determined that monoclonal antibody (MAb) binding to epidermal growth factor-like domains 4–6 of TM increased microtubule formation. Addition of thrombin, which binds to these EGF-like domains, decreased the augmented microtubule formation. We also investigated TM's involvement in EC invasion in vitro. Binding to the lectin-like domain of TM by MAb decreased invasion of HUVECs. Finally, we determined that binding to TM's extracellular EGF-like domains affected the ability of HUVECs to adhere to each other. We conclude that TM, specifically the EGF- and lectin-like domains of TM, is involved in angiogenesis by regulating the ability of endothelial cells to proliferate, invade, and adhere to each other. Funding: NIH Grant # P20RR-16461, McKay Urology Endowment Fund, and the Winthrop University Research Council.