Endo-β-1,4-D-mannanases (β-mannanase; EC 3.2.1.78) are endohydrolases that participate in the degradation of hemicellulose, which is closely associated with cellulose in plant cell walls. The β-mannanase from Trichoderma reesei (Man5A) is composed of an N-terminal catalytic module and a C-terminal carbohydrate-binding module (CBM). In order to study the properties of the CBM, a construct encoding a mutant of Man5A lacking the part encoding the CBM (Man5AΔCBM), was expressed in T. reesei under the regulation of the Aspergillus nidulans gpdA promoter. The wild-type enzyme was expressed in the same way and both proteins were purified to electrophoretic homogeneity using ion-exchange chromatography. Both enzymes hydrolysed mannopentaose, soluble locust bean gum galactomannan and insoluble ivory nut mannan with similar rates. With a mannan/cellulose complex, however, the deletion mutant lacking the CBM showed a significant decrease in hydrolysis. Binding experiments using activity detection of Man5A and Man5AΔCBM suggests that the CBM binds to cellulose but not to mannan. Moreover, the binding of Man5A to cellulose was compared with that of an endoglucanase (Cel7B) from T. reesei.