Extracellular matrix proteins are thought to provide a rigid mechanical anchor that supports and guides migrating and rolling cells(1-4). Here we examine the mechanical properties of the extracellular matrix protein tenascin by using atomic-force-microscopy techniques. Our results indicate that tenascin is an elastic protein. Single molecules of tenascin could be stretched to several times their resting length. Force-extension curves showed a saw-tooth pattern, with peaks of force at 137 pN. These peaks were similar to 25 nm apart. Similar results have been obtained by study of titin(5). We also found similar results by studying recombinant tenascin fragments encompassing the 15 fibronectin type III domains of tenascin. This indicates that the extensibility of tenascin map be due to the stretch-induced unfolding of its fibronectin type III domains. Refolding of tenascin after stretching, observed when the force was reduced to near zero, showed a double-exponential recovery with time constants of 42 domains refolded per second and 0.5 domains per second. The former speed of refolding is more than twice as fast as any previously reported speed of refolding of a fibronectin type III domain(6,7). We suggest that the extensibility of the modular fibronectin type III region may be important in allowing tenascin-ligand bonds to persist over long extensions. These properties of fibronectin type III modules may be of widespread use in extracellular proteins containing such domain(8,9).