Type XII and type XIV collagens are closely related homotrimeric and disulfide-bonded extracellular matrix constituents that contain two short triple helical domains near theC-terminus and a very large modular non-triple helical domain at theN-terminus. This domain contains multiple fibronectin type III repeats, von Willebrand factor A-like modules and a module homologous to the N-terminal region of thrombospondin. When visualized by rotary shadowing electron microscopy they have a cross shape with a thin collagenous tail, 75 nm in length, connected to a central globule from which three arms are sticking out (from 60 to 90 nm depending on the collagen type and the splicing variant). Collagens XII and XIV have been localized around collagen I-containing fibrils in dense connective tissues. Despite the presence in their primary structure of modules and sequences characteristic of adhesive glycoproteins, these molecules do not appear to have cell adhesion activity. They appear rather to play a role in modulating the biomechanical properties of collagen-containing tissues by mediating deformability of fibril bundles. Collagen XIV also serves as a ligand to procollagenN-proteinase and may contribute in this way to sequester this activity around the collagen fibrils.