The cellulase and beta-mannanase activities of Endoglucanase III from Trichoderma reesei can be attributed to the same active site. Reversible and irreversible inhibitors affect similarly both activities. Galactomannan is a competitive substrate for a specific endoglucanase substrate. Km values are similar for mannan and carboxymethylcellulose but kcat values are ten times higher for the latter.