Nineteen distinct types of collagen have been found in vertebrates. Studies with newly discovered collagens have lead us to propose two new subgroups of collagens, namely membrane-associated collagens, including type XIII and XVII collagens, and a subgroup formed by the homologous collagens XV and XVIII. Type XIII collagen is found ubiquitously in the matrix, and in view of its plasma membrane localization it may function in cell adhesion. The new homologous collagens XV and XVIII are characterized by a collagenous sequence with frequent interruptions and large amino and carboxyterminal noncollagenous domains. Type XVIII collagen chains have three variant amino-terminal ends and one of these variants is characterized by a new cysteine-rich sequence motif, termed the fz sequence. A key enzyme of collagen biosynthesis is prolyl 4-hydroxylase consisting of two alpha and two beta subunits. The beta subunit is a multifunctional polypeptide with several distinct activities. Furthermore, two types of alpha subunit have been identified resulting in enzyme tetramers with highly similar properties with some interesting differences.