Growth of anchorage-dependent cells requires both soluble mitogens and insoluble extracellular matrix molecules such as fibronectin. Soluble growth factors activate chemical signaling pathways and stimulate proliferation by binding to transmembrane receptors. Insoluble fibronectin also binds to cell-surface receptors; however, it is thought to act primarily via effects on the cytoskeleton and cell shape. We recently demonstrated that cell spreading on surface-adsorbed fibronectin activates the Na/H antiporter and that inhibition of this chemical-signaling pathway suppresses growth. We now show that insoluble fibronectin activates the Na/H antiporter by clustering and immobilizing integrin alpha 5 beta 1, independent of effects on cell shape. These results show that an extracellular matrix receptor can behave similarly to a growth factor receptor to activate a signaling pathway implicated in growth control.