AbstractWe have examined the hypothesis that glycolipids might serve as receptors for the cell surface glycoprotein fibronectin using three different biological assay systems. We find that purified solubilized gangliosides inhibit fibronectin‐mediated hemagglutination, cell spreading, and restoration of a normal morphologic phenotype to transformed cells. The inhibition is dose‐dependent and competitive; hemagglutination by 2 μg/ml fibronectin is half‐maximally inhibited by less than 1 μM gangliosides. The most effective ganglioside inhibitors generally contain the most sialic acid residues. The isolated oligosaccharide portions of gangliosides retain this inhibitory activity and the oligosaccharides with more sialic acid are more effective inhibitors.A series of other lipids or ganglioside constituents are either less effective or without detectable activity. The more active of these lipids are the more negatively charged phospholipids such as phosphatidyl serine and phosphatidyl inositol.Our results support the hypothesis that the “receptors” for fibronectin on the cell surface either consist of or contain gangliosides or other negatively charged lipids.