Some properties of purified endo-1,4-beta-D-xylanase (1,4-beta-D-xylan xylanohydrolase, EC 3.2.1.8) from the ligniperdous fungus Trametes hirsuta were investigated. The enzyme was stable between pH 4.0 and 8.0 were optimum activity at pH 5.0--5.5. The temperature optimum was 50 degrees C and the enzyme was stable for up to 30 min at 45 degrees C; however, it was denatured at higher temperatures. The Km for 4-O-methylglucurono-D-xylan was 6.36.10(-3) equivalents of D-xylose per litre, the activation energy was 28 kJ mol-1. The molecular weight determined by means of gel chromatography was 22000--24000. The enzyme was activated by Ca2+ and inhibited by Ag+ and Hg2+. On the basis of the effect of 2-hydroxy-5-nitrobenzyl bromide. N-bromosuccinimide and N-acetylimidazole it may be assumed that trytophan and possibly tyrosine residues influence the enzyme catalysis.