NOVELTY - Hydrolysis of cellulose containing biomass involves: pretreating cellulose containing biomass; and saccharifying pretreated cellulose containing biomass to convert into glucose. In the saccharification step, celluloses or cellulose-containing composition is added to the pretreated cellulose containing biomass. The celluloses or cellulose-containing composition comprises beta -glucosidase (BG) and another cellulose; where less than 10 mg cellulose (on protein) is added per gram cellulose present in the cellulose containing biomass (on dry matter).    USE - For hydrolysis of cellulose containing biomass (claimed).    ADVANTAGE - The method for the hydrolysis of cellulose containing biomass reduces operational costs such as enzyme costs, reactor costs and energy costs. The enzyme costs can be reduced by using less enzyme and at same time maintaining high production results such as high levels of sugars, ethanol, biogas or other desired products, all compared to known hydrolysis methods. The reactor cost can be reduced by selecting method conditions which result in lower total reactor volume compared to known hydrolysis methods. The energy savings can be obtained by lower need of energy supply to the method. By the addition of oxygen it is possible to attain many method advantages, including optimal temperature conditions, reduced method time, reduced dosage of enzyme, re-use of enzymes, higher yields and other method optimizations, resulting in reduced costs. The enzyme reduction in saccharification processes is possible without lower glucose yield by using lower amount of celluloses in combination with adapted mutual ratio of the different enzymes present. The method produces glucose in high yield.    DETAILED DESCRIPTION - Hydrolysis of cellulose containing biomass involves: pretreating cellulose containing biomass; and saccharifying pretreated cellulose containing biomass to convert into glucose. In the saccharification step, celluloses or cellulose-containing composition is added to the pretreated cellulose containing biomass. The celluloses or cellulose-containing composition comprises beta -glucosidase (BG) and another cellulose; where less than 10 mg cellulose (on protein) is added per gram cellulose present in the cellulose containing biomass (on dry matter). The amount of BG is at least 4 wt.% of the celluloses (on protein) or the amount of BG is at least 4 wt.% of the protein present in the celluloses or cellulose-containing composition added, preferably the amount of BG is at least 4 wt.% of the celluloses (on protein).