Tenascin-X (TNX) is a large 450 kDa extracellular matrix protein expressed in a variety of tissues including skin, joints and blood vessels. Deficiency of TNX causes a recessive form of Ehlers-Danlos syndrome characterized by joint hypermobility, skin fragility and hyperextensible skin. Skin of TNX deficient patients shows abnormal elastic fibers and reduced collagen deposition. The mechanism by which TNX deficiency leads to connective tissue alterations is unknown. Here we report that C-terminal domains of human TNX bind to major dermal fibrillar collagens and tropoelastin. We have mapped these interactions to the fibronectin type III repeat 29 (FNIII29) and the C-terminal fibrinogen domain (FbgX) of TNX. In addition we found that FNIII29 of TNX accelerates collagen fibrillogenesis in vitro. We hypothesize that TNX contributes to matrix stability and is possibly involved in collagen fibril formation.