For paperboard used to produce packaging, treatment of its fiber constituents with commercial enzymes can improve its compressive strength under cycling and high-humidity conditions. Xylanase that selectively removes pulp xylan (ca. 2% of the pulp by mass) yielded most of the observed beneficial effects, which did not appear related to the packing of the fiber network in the sheet or to the uptake of moisture at high humidity. There was also a marginal increase in the drainability of the pulp slurry, which may increase the rate of papermaking. Although endoglucanase activity also enhances certain pulp properties, there was little benefit in adding it to the xylanase treatment. (C) 2004 Wiley Periodicals, Inc.