In the solid tumor the microenvironment is the space limited by the basement membrane of the microvessels and the neoplastic cells membrane. It includes the stroma and a liquid phase, the tumor interstitial fluid (TIF). We developed a method to sample TIF in vivo and found it rich in prostaglandins. In the rabbit cornea PGE1 induces neovascularization and acts as an angiogenesis factor. Before angiogenesis appears the ganglioside content of the cornea doubles with sharp reduction of the GM3/GD3 ratio. These gangliosides are not angiogenic but they influence the endothelial cell behavior. In particular when the PGE1 dose is insufficient to induce angiogenesis, enrichment of corneal tissue with GD3 or GM1 stimulates angiogenesis. However, when the corneal tissue is enriched with GM3, doses of PGE1, normally angiogenic fail to do so. The same was observed when bFGF substituted PGE1 as an angiogenesis trigger. The gangliosides tested acted as modulators of the angiogenic response by promoting the angiogenic capacity of molecules, such as PGE1 or bFGF, normally present in the tissue microenvironment. Several neoplastic cells, especially melanomas, shed gangliosides in the microenvironment. Their modulatory effect on angiogenesis may influence metastatic and/or primary tumor growth.