A particulate preparation from 6-day-old seedlings of wheat (Triticum aestivum L.) was found to contain a xylosyltransferase (XylTase) which incorporated xylose (Xyl) from UDP-xylose into exogenous beta-(1-->4)-xylooligosaccharides with 2-aminopyridine-derivatized reducing end groups. High-performance liquid chromatographic analysis showed that the chain elongation of pyridylaminated beta-(1-->4)-xylotriose (Xyl3-PA) occurred by attachment of a series of one, two, or three xylosyl residues, depending on substrate concentrations and reaction times. Methylation analysis and beta-xylosidase digestion of the newly synthesized Xyl4-PA confirmed that the xylosyl residues were incorporated through beta-(1-->4)-linkages. The enzyme was maximally active at pH 6.8 and 20 degrees C, and required Triton X-100, which enhanced activity 5-fold at a concentration of 0.05-2%. Divalent ions, including Mn2+ and Mg2+, did not affect activity. Enzyme activity increased with increasing polymerization of xylosyl residues of the acceptor substrates: for instance, Xyl5-PA was almost 7 times as efficient as Xyl2-PA. The apparent Michaelis constants of the enzyme for Xyl3-PA and UDP-xylose were 13.5 and 7.9 mM, respectively. The enzyme also catalyzed incorporation of radioactive sugars (Xyl together with a small portion of L-arabinose) from UDP-[14C]xylose into higher beta-(1-->4)-xylooligosaccharides (degree of polymerization > 7) with or without (4-O-methyl-)glucuronosyl side chains at activities comparable to those observed for pyridylaminated xylooligosaccharides, and into several heteroxylans but with much lower efficiency. Enzymatic hydrolysis of the product with a beta-xylanase degraded it into mainly xylobiose, providing further evidence that the xylosyl residues are incorporated through beta-(1-->4)-linkages.