Basic fibroblast growth factor (bFGF) binds to heparin-like molecules present in the extracellular matrix (ECM) of transformed fetal bovine aortic endothelial GM 7373 cells. Binding of bFGF to ECM can be competed by heparin or heparan sulfate, and ECM-bound bFGF can be released by treating the cells with heparinase or heparatinase. After binding to ECM, bFGF is slowly released into the medium in a biologically active form, as shown by its capacity to induce an increase of cell-associated plasminogen activator activity and cell proliferation. The increase is prevented upon removal of ECM-bound bFGF by a neutral 2 M NaCl wash. Soluble heparin and heparan sulfate reduce the amount of ECM-bound bFGF released into the medium, possibly competing with ECM polysaccharides for heparinase-like enzymes produced by endothelial cells, suggesting that these enzymes are involved in the mobilization of ECM-bound bFGF.