The inhibition effect of cellobiose on the initial stage of hydrolysis when cellobiohydrolase Cel 7A and endoglucanases Cel 7B, Cel 5A, and Cel 12A from Trichoderma reesei were acting on bacterial cellulose and amorphous cellulose that were [H-3] labeled at the reducing end was quantified. The apparent competitive inhibition constant (K-i) for Cel 7A on [H-3]-bacterial cellulose was found to be 1.6+/-0.5 mM, 100-fold higher than that for Cel 7A acting on low-molecular-weight model substrates. The hydrolysis of [H-3]-amorphous cellulose by endoglucanases was even less affected by cellobiose inhibition with apparent K-i values of 11 +/- 3 mM and 34 +/- 6 mM for Cel 713 and Cel 5A, respectively. Contrary to the case for the other enzymes studied, the release of radioactive label by Cel 12A was stimulated by cellobiose, possibly due to a more pronounced transglycosylating activity. Theoretical analysis of the inhibition of Cel 7A by cellobiose predicted an inhibition analogous to that of mixed type with two limiting cases, competitive inhibition if the prevalent enzyme-substrate complex without inhibitor is productive and conventional mixed type when the prevalent enzyme-substrate complex is nonproductive. (C) 2004 Wiley Periodicals, Inc.