Using recombinant DNA techniques, an Aspergillus nidulans multicopy transformant for the gene xlnB coding for the minor X(24) xylanase has been constructed. When grown on glucose as sole carbon source this transformant secretes 114 U of xylanase (mg protein)(-1). In this culture condition, X(24) is the only xylanase secreted and the predominant protein in the culture filtrate. This strategy has been used to purify the X(24). enzyme to homogeneity. The purified xylanase showed a single band on sodium dodecyl sulphate/polyacrylamide gel electrophoresis with a molecular mass of 24 kDa and had an isoelectric point of approximately 3.5. The enzyme was a non-debranching endo-1,4-beta-xylan xylanohydrolase highly specific for xylans and showed optimal activity at pH 5.5 and 52 degrees C. The X(24) xylanase had a Michaelis constant, K-m, of 12.43 mg oat spelt xylan ml(-1) and a V-max of 1639 mu mol min(-1) (mg protein)(-1).