In the presence of xylan, Streptomyces sp. strain S38 secretes three xylanases (Xy11, Xy12, and Xy13) th:lt were purified to protein homogeneity and characterized. When used in bleach boosting tests on kraft hardwood and softwood, Xy11, a family-11 enzyme, was more effective than Xy12 and Xy13 that belonged to family-10. Xy11 was fully responsible for the biodelignification potential of the culture supernatants with a minimal effective amount of 10 IU per gram of dry pulp for both softwood and hardwood pulp. Complete conventional CEDED bleaching sequences showed that enzymatic pretreatment (20 IU/g dry pulp) could result in active chlorine savings of 8.6 and 4.9 kg/ton of dry pulp with hardwood and softwood, respectively. The purified enzymes were totally devoid of cellulase activity on CM-cellulose and their activities were optimal at about 60 degrees C and pH 6. Moreover, the V-max value of Xy11 at 50 degrees C measured on birchwood xylan (5,700 mu moles/min/mg prot.) was significantly higher than those of Xy12 and Xy13 whereas their k(m) values were similar. Their half-lives at 50 degrees C were larger than 16 h but sharply decreased at 60 degrees C where the family-11 Xy11 was less stable (t(1/2)(60 degrees C) = 10 min) than both family-10 enzymes Xy12 (t(1/2) = 30 min) and Xy13 (t(1/2)(60 degrees C) = 70 min). (C) 2000 Elsevier Science Inc. All rights reserved.