Application of cellulolytic enzymes for upgrading of secondary fibers has extensively been studied during the last decades. In many cases, research has been carried out with industrial enzyme preparations which often consist of several enzyme components. The presence of various enzymes makes the determination of the role of each of them difficult in changes of pulp and paper properties. Therefore, investigations using single enzyme preparations are necessary to understand and optimize this complex process. One of the possibilities to produce individual cellulase components is repression of undesired genes. In this work, two recombinants of Trichoderma reesei strain QM9414 were used to produce intact Ce17B (endoglucanase 1, EGI) and the catalytic core of Ce17B selectively under glucose repression of other cellulases. For homologous expression in glucose medium the gpdA promoter from Aspergillus nidulans was used. Culture filtrates containing the intact Ce17B or Ce17B core were applied for secondary fiber treatment. The treatments caused improved drainage as shown by 16-17% decrease of the Schopper-Riegler (SR) values. The effects of cellulase treatment on pulp and paper properties showed that presence or absence of a cellulose-binding module (CBM) on Ce17B does not play a determinative role. (c) 2006 Elsevier Ltd. All rights reserved.