Tenascin-X is known as a heparin-binding molecule, but the localization of the heparin-binding site has not been investigated until now. We show here that, unlike tenascin-C, the recombinant fibrinogen-like domain of tenascin-X is not involved in heparin binding. On the other hand, the two contiguous fibronectin type III repeats b10 and b11 have a predicted positive charge at physiological pH, hence a set of recombinant proteins comprising these domains was tested for interaction with heparin. Using solid phase assays and affinity chromatography, we found that interaction with heparin was conformational and involved both domains 10 and 11. Construction of a three-dimensional model of domains 10 and 11 led us to predict exposed residues that were then submitted to site-directed mutagenesis. In this way, we identified the basic residues within each domain that are crucial for this interaction. Blocking experiments using antibodies against domain 10 were performed to test the efficiency of this site within intact tenascin-X. Binding was significantly reduced, arguing for the activity of a heparin-binding site involving domains 10 and 11 in the whole molecule. Finally, the biological significance of this site was tested by cell adhesion studies. Heparan sulfate cell surface receptors are able to interact with proteins bearing domains 10 and 11, suggesting that tenascin-X may activate different signals to regulate cell behavior.