Endohydrolysis of (1->4)-beta-D-glucosidic linkages in cellulose, lichenin and cereal beta-D-glucans. Optimum pH is 9.0. Alkalophilic Bacillus sp strain 1139 is not a true cellulolytic microorganism because the enzyme is unable to hydrolyze native cellulose. Belongs to the glycosyl hydrolase 5 (cellulase A) family.