Fifty actinomycete strains were screened for the production of mannanase activity during growth in both. liquid and solid media. Streptomyces scabies CECT 3340 and Streptomyces ipomoea CECT 3341 were selected for their ability to produce high levels of mannanase (294.3 U/l and 242.9 U/l, respectively) during growth in liquid culture. beta-Mannosidase (15.3 U/l) and alpha-galactosidase (7.7 U/l) activities were also detected in culture filtrate from S. scabies CECT 3340. Highest levels of mannanase activity for S. scabies CECT 3340 were achieved in media containing locust bean gum and asparagine (4.8 U mg-l protein) whilst in S. ipomoea CECT 3341 greatest activity was detected in media containing locust bean gum and yeast extract (13.2 U mg(-1) protein). No carboxymethylcellulase activity was detected. In big; bleaching experiments, enzyme treatment, carried out with mannanase activity produced by S. ipomoea CECT 3341, followed by alkaline extraction of pine kraft pulp resulted in the release of colour (A(465), 0.69) and chromophoric material from the pulp (A(237), 12.9; A(254), 6.9 and A(280), 6.7). The ability of this enzyme complex to improve the bleaching of pine kraft pulps was also shown by a pulp brightness increase (2.4 units ISO) and a reduction in kappa number (from 21.4 units to 20.1 units) with the absence of variations on the viscosity values.