The modification of fibre surface by an enzymatic pretreatment was studied by analyzing the hydrolysis products released from the pulps and the molar mass distribution of the alkali-extracted residual lignin. In kraft pulp, the reprecipitated or readsorbed alkali-resistant xylan appears to form a physical barrier to the extraction of residual lignin from the fibres. The treatment of kraft pulp with hemicellulases removes some of this xylan and renders the fibre structure more- permeable. The increased permeability allows the passage of lignin or lignin-carbohydrate molecules in higher amounts and of higher molecular masses in the subsequent chemical extraction. The improved extractability of lignin leads to higher brightness of pulp, or can alternatively be exploited to reduce the consumption of bleaching chemicals.