This study demonstrates the potential of laccase-facilitated grafting of amino acids to high-lignin content pulps to improve their physical properties in paper products. Research studies have recently reported that increases in anionic fiber charge can improve strength properties of paper. In an effort to increase carboxylic acid groups, we developed a unique two-stage laccase grafting protocol in which fibers were initially treated with laccase followed by grafting reactions with amino acids. The bulk acid group content was measured, and a variety of amino acids including glycine (Gly), phenylalanine (Phe), serine (Ser), arginine (Arg), histidine (His), alanine (Ala), and aspartic acid (Asp) were examined. The effects of optimizing laccase dose, and amino acid structures, on fiber modification chemistry were studied. Histidine provided the best yield of acid groups on pulp fiber, and was used for the preparation of handsheets for physical strength testing. Laccase-histidine treated pulp showed an increase in strength properties of the resulting paper. (C) 2008 Elsevier Inc. All rights reserved.