Abstract An endoglucanase-cellobiohydrolase from Trichoderma reesei culture fluids was purified by means of preparative isoelectric focusing. The cellulase complex had a common apparent isoelectric point (pI) of 3.8. Beyond this pI, the electrophoretic mobilities of endoglucanase and cellobiohydrolase were different under conditions of titration curves. The effect of this endoglucanase-cellobiohydronalase complex on Sinapis cellulose microfibril ultranstructure was observed by transmission electron microscopy after metal shadowing of the specimen. By the action of this cellulase complex, the microfibril structure was converted into an amorphous form of cellulose. Moreover, the hydrolase complex induced visible cross-fractures within the cellulose microfibril structure. The mean cellulose microfibril lenght of 1.2 μm was reduced to 0.9 μm in the presence (12 h) of this cellulase complex by the formation of shorter microfibril fragments.