An enzyme hydrolyzing beta-1,4 bonds in cellulose acetate was purified 10.5-fold to electrophoretic homogeneity from a culture supernatant of Neisseria sicca SB, which assimilate cellulose acetate as the sole carbon and energy source. The enzyme was an endo-1,4-beta-glucanase, to judge from the substrate specificity and hydrolysis products of cellooligosaccharides, we named it endo-1,4-beta-glucanase I (EG I). Its molecular mass was 50 kDa, 9 kDa larger than EG II from this strain, and its isoelectric point was 5.0. Results of N-terminal and inner-peptide sequences of both enzymes, and a similarity search, suggested that EG I contained a carbohydrate-binding module at the N-terminus and that EG II lacked this module. The pH and temperature optima of EG I were 5.0-6.0 and 45 degrees C. It hydrolyzed water-soluble cellulose acetate (degree of substitution, 0.88) and carboxymethyl cellulose. The Km and Vmax for these compounds were 0.296% and 1.29 micromol min(-1) mg(-1), and 0.448% and 13.6 micromol min(-1) mg(-1), respectively. Both glucanases and cellulose acetate esterase from this strain degraded water-insoluble cellulose acetate synergistically.