A new insoluble dye substrate for xylan degrading enzymes was prepared. Xylan was dyed with Cibacron blue 3GA and cross-linked by 1,4-butanedioldiglycidylether. The substrate was stable at 25 degrees C for 60 h. Xylanase assay by the substrate was more sensitive than conventional dinitrosalicylic acid method and was not interfered with by a reducing sugar. The substrate could be used for simple and sensitive measurement of xylanase activities and preparation of agar plates for screening of xylanase producing microorganisms. (C) 1997 Elsevier Science Ireland Ltd.