Many collagen fibrils have been shown to be heterotypic, i.e. composed of more than one collagen type. Fibrils containing type I collagen as the major constituent do also contain, at least in some tissues, type III and type V collagens. Fibrils containing type II collagen have been shown to also contain type XI collagen. The type I, II, III, V and XI collagen molecules are very similar and are clearly derived from a single ancestral gene. However their processings are not identical. While collagen types I and II have a N-propeptide which is cleaved for their insertion in the fibrils, collagen types V and XI keep a N-terminal extension which must include, based on the cDNA derived structures, a short triple helix and a globular domain. They are thought to contribute to the control of fibril lateral growth and diameter. Other collagens are associated with fibrils without having the long triple uninterrupted triple helix characteristic of collagen types I, II, III, V and XI. Type IX collagen has been shown to be covalently cross-linked to type II collagen and to lay at or near the surface of fibrils, with a triple helical arm projecting in the extrafibrillar space a globular N-terminal domain. Type XII collagen is found in type I collagen containing matrices and contains a triple helical domain homologous to the type IX COL1 domain. This suggests a similar function.(ABSTRACT TRUNCATED AT 250 WORDS)