This enzyme hydrolyzes 1,4-beta-D-glucosidic linkages of cellulose. Weak activity against carboxymethylcellulose, bacterial microcrystalline cellulose and barley beta-glucan. Has also weak endoglucanase activity. Hydrolyzes glucosidic bonds with inversion of anomeric configuration. Hydrolysis of (1->4)-beta-D-glucosidic linkages in cellulose and cellotetraose, releasing cellobiose from the non-reducing ends of the chains. Belongs to the glycosyl hydrolase 6 (cellulase B) family.