Genetic ablation of angiopoietin-1 (Ang-1) or of its cognate receptor, Tie2, disrupts angiogenesis in mouse embryos. The endothelial cells in growing blood vessels of Ang-1 knockout mice have a rounded appearance and are poorly associated with one another and their underlying basement membranes (Dumont, D. J., Gradwohl, G., Fong, G. H., Puri, M. C., Gertsenstein, M., Auerbach, A., and Breitman, M. L. (1994) Genes Dev. 8, 1897–1909; Sato, T. N., Tozawa, Y., Deutsch, U., Wolburg-Buchholz, K., Fujiwara, Y., Gendron-Maguire, M., Gridley, T., Wolburg, H., Risau, W., and Qin, Y. (1995) Nature 376, 70–74; Suri, C., Jones, P. F., Patan, S., Bartunkova, S., Maisonpierre, P. C., Davis, S., Sato, T. N., and Yancopoulos, G. D. (1996) Cell 87, 1171–1180). It is therefore possible that Ang-1 regulates endothelial cell adhesion. In this study we asked whether Ang-1 might act as a direct substrate for cell adhesion. Human umbilical vein endothelial cells (HUVECs) plated for a brief period on different substrates were found to adhere and spread well on Ang-1. Similar results were seen on angiopoietin-2 (Ang-2)-coated surfaces, although cells did not spread well on Ang-2. Ang-1, but not Ang-2, supported HUVEC migration, and this was independent of growth factor activity. When the same experiments were done with fibroblasts that either lacked, or stably expressed, Tie2, results similar to those with HUVECs were seen, suggesting that adhesion to the angiopoietins was independent of Tie2 and not limited to endothelial cells. Interestingly, when integrin-blocking agents were included in these assays, adhesion to either angiopoietin was significantly reduced. Moreover, Chinese hamster ovary-B2 cells lacking the α5 integrin subunit did not adhere to Ang-1, but they did adhere to Ang-2. Stable expression of the human α5 integrin subunit in these cells rescued adhesion to Ang-1 and promoted an increase in adhesion to Ang-2. We also found that Ang-1 and Ang-2 bind rather selectively to vitronectin. These results suggest that, beyond their role in modulating Tie2 signaling, Ang-1 and Ang-2 can directly support cell adhesion mediated by integrins.