D-Xylose, once thought to be restricted to polysaccharides of plants and microorganisms, has recently been found in a number of macromolecules of animal origin. This pentose occupies a unique position in chondroitin through a xylosyl-serine linkage ( Gregory et al ., 1964 ; Lindahl and Rodén, 1965; Rodén and Lindahl, 1965). Free xylosyl-serine has been isolated from human urine ( Tominaga et al ., 1965 ). Xylose ∗ ∗ The sugars are of the D-configuration. has been reported to be a constituent of mucopolysaccharides of mammalian brain ( Wardi et al ., 1965 ), and of glycoproteins from human lung ( Masamune et al ., 1957 ), cancerous ascitic fluid ( Sugimoto, 1956), placenta ( Tomoda and Murayama, 1965) and aorta ( Klemer and Burbaum, 1965), and from cockroach plasma (Lipke et al ., 1965). UDP-Xylose, the probable precursor of such glycosidically-bound xylose, has been isolated from the milk of sows and goats, and from cow colostrum ( Kobata and Zirô, 1965), while its biosynthesis by decarboxylation of UDP-glucuronic acid has been demonstrated in preparations from hen oviduct ( Bdolah and Feingold, 1965). The present communication describes a particulate system from the oviduct of laying hens, which catalyzes the transfer of xylose from UDP-xylose into glycoprotein.