Endothelial binding of heparin contributes to its local antithrombotic action and catabolism. However, it is uncertain whether heparin may be bound to a damaged or even de-endothelialized vessel surface. Therefore, the binding of 3H-heparin to cultured endothelial cell monolayer and extracellular matrix was studied. The binding reached equilibrium after 4 h. 3H-heparin bound to endothelium could be displaced by unlabelled heparin which competed for about 80% of binding. The binding became saturable when the concentration of heparin exceeded 30-times the therapeutical value. Approximately 6 X 10(11) binding sites for heparin per cm2 of endothelium were calculated. Heparin was bound not only to endothelial cells but also to extracellular matrix, even when it was exposed in the absence of cells. About 40% of binding sites were localized in the extracellular matrix fraction. A similar affinity of binding of 3H-heparin to complete endothelium, endothelial cells and extracellular matrix was estimated /Kd of almost 1 microM/. The binding of heparin to extracellular matrix should be considered in the interpretation of heparin interaction with endothelium.