1 Angiopoietin-1 (Ang1) is an angiogenic growth factor that binds to the Tie2 receptor on vascular endothelium, promoting blood vessel maturation and integrity. In the present study, we have investigated whether Ang1 also possesses anti-inflammatory properties by determining its effects on endothelial barrier function, neutrophil (PMN) adherence to endothelial cells (EC) and production of the PMN chemotactic factor interleukin-8 (IL-8). 2 Pretreatment of endothelial monolayers with Ang1 attenuated the permeability increase induced by thrombin in both lung microvascular cells and a human endothelial cell line. Similarly, Ang1 prevented the permeability-inducing effects of platelet-activating factor, bradykinin and histamine. 3 Pretreatment of EC with Ang1 also reduced the adherence of PMN to EC stimulated by thrombin. In contrast to its ability to counteract the increase in monolayer permeability brought about by various inflammatory agents, Ang1 did not affect the ability of histamine, PAF, or tumor necrosis factor-alpha to stimulate PMN adherence to EC. 4 In addition to its ability to inhibit PMN adherence, Ang1 diminished IL-8 production from EC challenged with thrombin in a concentration-dependent manner. 5 When EC were preincubated with the specific Rho kinase (ROCK) inhibitor Y-27632, we observed a reduction in PMN adherence in response to thrombin, as well as a decrease in thrombin-stimulated IL-8 production. Coincubation of monolayers with Y-27632 and Ang1 did not further attenuate the above-mentioned responses. However, Ang-1 failed to inhibit the activation of RhoA in response to thrombin, suggesting that inhibition of EC adhesiveness for PMN and IL-8 production by Ang1 does not result from reduced ROCK activation. 6 We conclude that Ang1 can counteract several aspects of the inflammatory response, including endothelial permeability, PMN adherence to EC as well as inhibition of IL-8 production by EC.