A series of chitosans with various molecular weights from 1.2 X 10(3) to 30.0 X 10(4) were prepared by oxidative degradation with H2O2 and characterized by IR, C-13-NMR, and gel permeation chromatography. Their carboxylic contents increased with a decrease in molecular weight (M-w). The moisture-absorption and moisture-retention capacities of resulting chitosans were dependent on both the molecular weight and the degree of deacetylation (DD). Microcalorimetry was first used to study the kinetics of action of the chitosans on a strain of Staphylococcus aureus at pH 7. The antibacterial activity of the water-soluble chitosan against S. aureus, Escherichia coli, and Salmonella typhi was evaluated by the conventional agar plate method at pH 7. The water-sofuble product with M-w of 0.45 X 10(4) from initial chitosan of DD of 90% showed high moisture-absorption and moisture-retention capacities, and -2% concentration can completely inhibit the growth of these bacteria.