This enzyme catalyzes the exohydrolysis of 1,4-beta-glucosidic linkages in cellulose with a preference for amorphous or crystalline cellulose over carboxymethyl cellulose. Hydrolysis of (1->4)-beta-D-glucosidic linkages in cellulose and similar substrates, releasing cellobiose from the reducing ends of the chains. Inhibited by cellobiose and lactose, but not by glucose. Belongs to the glycosyl hydrolase 48 (cellulase L) family. The N-terminal sequence shown here has been extracted from PDB entry 1L1Y.