An extracellular 105-kDa beta-mannosidase (beta-D-mannoside-mannohydrolase, E.C. 3.2.1.25) was purified to homogeneity from culture filtrate of Trichoderma reesei. Specific activity of the beta-mannosidase toward p-nitrophenyl-beta-D-mannopyranoside was 3.2 U/mg at the optimal pH 3.5 (K-m = 0.12 mM, k(CAT) = 2.95 x 10(-3) mu mol min/mu g. An additional beta-galactomannan (GM) binding site of the enzyme was found on the basis of kinetic studies. The enzyme GM dissociation constant (K-D) was 1.21 mg/ml. beta-1,4-mannooligosaccharides inhibited the binding of the enzyme to galactomannan. The inhibition constant of the sorption decreased with increasing of the beta-1,4-mannooligosaccharide length. Mannose, the competitive inhibitor of the beta-mannosidase in hydrolysis of p-nitrophenyl-beta-D-mannopyranoside, did not inhibit sorption of the enzyme on beta-GM. Chitin, xylan, raw starch, and microcrystalline cellulose had no affinity to the beta-mannosidase. The enzyme hydrolyzed beta-1,4-mannooligosaccharides with the rate depending on the chain length and liberated mannose from soluble and insoluble fractions of beta-GM from locust beans with initial rates of 0.3 and 0.05 mu mol min/ml U, respectively.