Acceptor affinities of UDP-D-xylose:proteoglycan core protein β-D-xylosyltransferase (XT) recognition signals in synthetic L-APP and L-APLP2 homologous peptides were determined. The Michaelis–Menten constants (KM) of the L-APP peptide TENEGSGLTNIK and the L-APLP2 peptide SENEGSGMAEQK were 20.1 and 18.9 μM, respectively. Therefore, the peptides proved to be as good acceptors for XT as the bikunin aminoterminus homologous peptide (KM = 22 μM). Due to the occurrence of L-APP and L-APLP2 transcripts in human brain tissue, XT activity was measured in human liquor cerebrospinalis. Mean values were calculated as 0.22 mU/L in males and 0.47 mU/L in females without disturbance of blood–brain barrier. In addition, in homogenized rat brain tissue a mean XT activity of 0.75 mU/L was determined. Furthermore, XT activity was investigated in 21 different human cell lines. In 7 cell lines an enzyme activity was not detected in either extracellular space or cytoplasma. Our findings indicate that XT is not ubiquitously expressed in human cell types.