The mechanisms of chain selection and assembly of type IX collagen, a heterotrimer alpha 1(IX)alpha 2(IX)alpha 3(IX), must differ from that of fibrillar collagens since it lacks the characteristic C-propeptide of these latter molecules. We have tested the hypothesis that the information required for this process is contained within the C-terminal triple helical disulfide-bonded region (LMW). The reassociations of the purified LMW fragments of pepsinized bovine type IX collagen were followed by the formation of disulfide-bonded multimers. Our data demonstrate that only three triple helical assemblies form readily, (alpha 1)3, (alpha 2)3 and alpha 1 alpha 2 alpha 3. The information required for chain selection and assembly is thus, at least in part, contained in the studied fragments. Molecular stoichiometries different from the classical heterotrimer may thus also form under certain conditions.