The endoglucanase CelB isolated from culture filtrates of Streptomyces lividans IAF9 has an M(r) of 36,000. With carboxymethyl cellulose as the substrate, the Vmax and Km values are 110 IU/mg of enzyme and 1.3 mg/ml, respectively. Comparison of primary amino acid sequences classifies CelB in the H family of cellulases.