Chitosan-N-2-hydroxypropyl trimethyl ammonium chloride (QTS) was prepared by reaction of chitosan with glycidyl trimethylammonium chloride, which was characterized by FTIR. QTS with different molecular weights (M-w 41.55 X 10(4), 9.02 X 10(4), 3.57 x 10(4), and 0.17 X 10(4)) showed biocidal activity on Staphylococcua aureus, Bacillus subtilis, Staphylococcua epidermidis, and Candida albicans. QTS with high molecular weight had high biocidal activity on the gram-positive bacteria, and the biocidal effect of QTS decreased with decreasing molecular weight from 9.02 X 10(4) to 0.17 x 10(4). QTS with M-w 41.55 x 10(4) exhibits slightly lower biocidal activity on Candida albicans than other QTS samples. However, no remarkable biocidal activity of QTS was found on gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa at the concentration up to 10 g L-1. Existence of cationic surfactant, amphoteric surfactant, nonionic surfactant, Ca2+ and Mg2+ had no remarkable effect on microbiocidal activity of these QTS samples. (c) 2006 Wiley Periodicals, Inc.