Mobilization of the capillary endothelium is one of the first events observed during angiogenesis, and the study of conditions that control or influence the mobilization of the endothelium in vitro has been assumed to offer information relevant to the understanding of angiogenesis in vivo. In vitro mobilization of the bovine capillary endothelium was substantially enhanced by addition of gangliosides to the culture medium. Optimal mobilization was obtained when the endothelium incorporated the gangliosides first and was then seeded on fibronectin anchored to collagen type I. Preincubation of the capillary endothelium with gangliosides, trisialoganglioside in particular, doubled the amount of fibronectin bound to the cells and enhanced the migration about 5-fold. 'Blockage' of ganglioside binding with cholera toxin or gamma-interferon substantially reduced migration. Rabbit corneas, treated in vivo with a variety of angiogenesis effectors to induce neovascularization, consistently showed an increase in sialic acid content just prior to the time the tissue would be penetrated by the capillaries. This finding was interpreted to indicate that an increment of the ganglioside content of the capillary endothelial cell membranes may play a determinant role in the mobilization of the capillary endothelium in vivo as shown here to take place in vitro. Since the formation of a tumor from a micrometastasis requires formation of new capillaries and highly metastasizing tumors very frequently have high levels of sialic acid on the cell surface, it is hypothesized that production and shedding of gangliosides from the surface of neoplastic cells may be a factor in promoting angiogenesis and metastatic growth.