Collagen is a trimer of three left-handed alpha-chains representing repeats of the motif Gly-X-Y, whereas (hydroxy)proline and (hydroxy)lysine residues are often found at positions X and Y. Selected hydroxylysine are further modified by the addition of galactose and glucose-galactose units. Collagen glycosylation takes place in the endoplasmic reticulum before triple helix formation and is mediated by beta(1-O)galactosyl- and alpha(1-2)glucosyltransferase enzymes. We have identified two collagen galactosyltransferases using affinity chromatography and tandem-MS protein sequencing. The two collagen beta(1-O)galactosyltransferases corresponded to the GLT25D1 and GLT25D2 proteins. Recombinant GLT25D1 and GLT25D2 enzymes showed a strong galactosyltransferase activity towards various types of collagen and towards the serum mannose-binding lectin MBL, which contains a collagen domain. Amino acid analysis of the products of GLT25D1 and GLT25D2 reactions confirmed the transfer of galactose to hydroxylysine residues. The GLT25D1 gene is constitutively expressed in human tissues, whereas the GLT25D2 gene is only expressed at low levels in the nervous system. The GLT25D1 and GLT25D2 enzymes are similar to CEECAM1, to which we could not attribute any collagen galactosyltransferase activity. The GLT25D1 and GLT25D2 genes now allow addressing the biological significance of collagen glycosylation and the importance of this post-translational modification in the etiology of connective tissue disorders.