The gene coding for a beta-mannanase was cloned homologously from Streptomyces lividans and its DNA sequence was determined. The fully secreted enzyme was isolated and purified from culture filtrates of the hyperproducing clone S. lividans IAF36 grown in mineral salt media containing galactomannan as the main carbon source. It had a molecular mass of 36 kDa and a specific activity of 876 i.u./mg of protein. Under the assay conditions used, the optimal enzyme activity was obtained at 58 degrees C and a pH of 6.8. The pI was 3.5. The kinetic constants of this mannanase determined with galactomannan as substrate were a Vmax. of 205 i.u./mg of enzyme and a Km of 0.77 mg/ml. Data from SDS/PAGE and Western blotting show that the cloned enzyme was identical to that of the wild-type strain.