In this work the effect of hemicellulases on birch kraft pulp was evaluated by following the amount of lignin leached from kraft pulp after enzymatic treatment. The difference in the amount of lignin leached by 0.01M NaOH solution from the xylanase-treated and untreated pulps can be regarded as a measure of the efficiency of the xylanase pretreatment prior to bleaching. An increase in the amount and average molar mass distribution of lignin extracted from the xylanase treated pulps was observed as compared with the reference pulps. In addition, different leaching kinetics of lignin were observed in the conventional and flow-through digested (FTD) pulps. The method can be used for evaluation of the increased bleachability of kraft pulps after enzymatic modifications of fibre surfaces.