Transmigration of leukocytes across the endothelial barrier is a tightly controlled process involving multiple steps, including rolling adhesion, firm adhesion, and then penetration of leukocytes through the endothelial monolayer. While the key molecular signals have been described in great detail, we are only just beginning to unveil the mechanical forces involved in this process. Here, using a microfabricated system that reports traction forces generated by cells, we describe forces generated by endothelial cells during monocyte firm adhesion and transmigration. Average traction force across the endothelial monolayer increased dramatically when monocytes firmly adhered and transmigrated. Interestingly, the endothelial cell that was in direct contact with the monocyte exhibited much larger traction forces relative to its neighbors, and the direction of these traction forces aligned centripetally with respect to the monocyte. The increase in traction force occurred in the local subcellular zone of monocyte adhesion, and dissipated rapidly with distance. To begin to characterize the basis for this mechanical effect, we show that beads coated with anti-ICAM-1 or VCAM-1 antibodies bound to monolayers could reproduce this effect. Taken together, this study provides a new approach to examining the role of cellular mechanics in regulating leukocyte transmigration through the endothelium.