The major alpha-glucuronidase of T. reesei Rut C-30 was purified by chromatographic methods The molecular and hydrolytic properties of the purified enzyme were studied. The enzyme had a molecular weight of 91,000 in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and a pi of 5.0-6.2 as determined by chromatofocusing. The pH optimum was pH 4.5-6.0 and the enzyme was stable for 24 h at 40 degrees C at pH 4.8-5.5. The purified alpha-glucuronidase preferred low-molecular-weight xylooligomers as substrate. The enzyme seemed to act almost exclusively on the bond between the terminal xylose at the nonreducing end of a xylose chain and the methyl glucuronic acid attached to it. Minor activity against long-chain glucuronoxylan was also detected. A significant enhancing effect of alpha-glucuronidase on the hydrolysis of glucuronoxylan by pure xylanases was observed.