Aspects of tumor-induced angiogenesis in vitro were examined using an assay involving collagen gel invasion by a surface monolayer of bovine endothelial cells under the influence of serum free conditioned medium produced by C6 cells, an experimentally derived rat glial tumor cell line. The effects of the polyanionic compound suramin, known to interfere with growth factor/cell signaling on this process were evaluated. Collagen gel invasion was quantified by adding C6 conditioned medium with or without various doses of suramin to monolayers of bovine aortic endothelial cells grown on type I collagen gels in transwell inserts. Cultures were monitored with phase-contrast microscopy. After various periods of incubation collagen gels were fixed, embedded in epoxy resin, and 1-micron thick sections were stained with toluidine blue. Additional cultures were used to evaluate the effects of C6 conditioned medium and suramin on endothelial cell proliferation, and on chemotaxis through 8-microns pores. C6 glioma cell conditioned medium induced large vessel endothelial cells to sprout into the underlying collagen matrix and subsequently form networks of capillary like tubes. Conditioned medium was also chemotactic and mitogenic for these cells. The addition of suramin to C6 glioma conditioned medium prevents tube formation in collagen gels, and inhibits both endothelial cell proliferation and chemotaxis in a dose dependent manner. These results suggest that glial tumor cell conditioned medium induces angiogenesis in large vessel endothelial cells in vitro via mechanisms which are disrupted by suramin, most likely involving tumor-derived growth factor release and/or endothelium-mediated matrix proteolysis.