T his study was designed to identify the set of functions activated in cultured endothelial cells by the hematopoietic growth factors, granulocyte colony-stimulating factor (G-CSF) and granulocyte macrophage-colony-stimulating factor (GM-CSF), and to compare them with those elicited by prototypic cytokines active on these cells. Moreover, indications as to the in vivo relevance of in vitro effects were obtained. G-CSF and GM-CSF induced endothelial cells to proliferate and migrate. In contrast, unlike appropriate reference cytokines (IL-1 and tumor necrosis factor, IFN-gamma), G-CSF and GM-CSF did not modulate endothelial cell functions related to hemostasis-thrombosis (production of procoagulant activity and of platelet activating factor), inflammation (expression of leukocyte adhesion molecule-1 and production of platelet activating factor), and accessory function (expression of class II antigens of MHC). Other colony-stimulating factors (IL-3 and macrophage-colony-stimulating factor) were inactive on all functions tested. In comparison to basic fibroblast growth factor (bFGF), G-CSF and GM-CSF induced lower maximal proliferation of endothelial cells, whereas migration was of the same order of magnitude. G-CSF and GM-CSF stimulated repair of mechanically wounded endothelial monolayers. Exposure to both cytokines induced shape changes and cytoskeletal reorganization consistent with a migratory phenotype. To explore the in vivo relevance of the in vitro effects of these cytokines on endothelium, we studied the angiogenic activity of human G-CSF in the rabbit cornea. G-CSF, but not the heat-inactivated molecule, had definite angiogenic activity, without any sign of inflammatory reactions. G-CSF was less active than bFGF. However, the combination of a nonangiogenic dose of bFGF with G-CSF resulted in an angiogenic response higher than that elicited by either individual cytokines. Thus, G-CSF and GM-CSF induce endothelial cells to express an activation/differentiation program (including proliferation and migration) related to angiogenesis.