Purified antibodies were prepared to UDP-D-xylose: core protein xylosyltransferase, the enzyme which initiates the formation of chondroitin sulfate chains in the course of proteoglycan biosynthesis in cartilage. The purified antibodies were conjugated to ferritin with a two-step glutaraldehyde procedure, and conjugates were then used to locate xylosyltransferase in fragments of embryonic cartilage cells. The results indicated that the enzyme is located within the cisternae of the rough endoplasmic reticulum. The distribution of the enzyme was similar to that of prolyl hydroxylase in the same cell fragments, suggesting that procollagen synthesis and initiation of chondroitin sulfate chains occur in the same regions of the rough endoplasmic reticulum.