Hordeum vulgare L, xylanase inhibitor (HVXI), an endoxylanase inhibitor with a protein structure, was purified to homogeneity from barley (Hordeum vulgare L.). HVXI is a nonglycosylated monomeric protein, with a molecular weight of approximate to 40,000 and a pI greater than or equal to 9.3. Although it inhibits different endoxylanases to a varying degree, the activities of an alpha -L-arabinofuranosidase and a beta -D-xylosidase were not inhibited. Apparently, HVXI occurs in two molecular forms. These characteristics and the N-terminal sequences of the composing polypeptides show that HVXI is homologous with Triticum aestivum L. xylanase inhibitor I, an endoxylanase inhibitor from wheat flour.