The present invention provides a purified, homogeneous plant enzyme that adds a .beta.-1,2-linked xylose to the .beta.-linked mannose on the N-linked oligosaccharides of storage glycoproteins. This .beta.1,2-xylosyltransferase was purified from the microsomal fraction of soybean cells approximately 51,000-fold. Also provided is an antibody recognizing this .beta.1,2-xylosyltransferase.