Silver nanoparticles were prepared by gamma-ray irradiation-reduction under simple conditions, i.e., air atmosphere, using chitosan as a stabilizer. The nanoparticles were spherical with an average size of 7-30nm as observed from TEM. The size decreased when chitosan concentration increased, while it increased with increasing gamma-ray dose and initial silver nitrate content. The obtained silver nanoparticles dispersed in a 0.5% (w/v) gamma-ray irradiated chitosan-aclueous acetic acid solution were stable for more than 3 months without tendency to precipitate. The silver nanoparticles exhibited antimicrobial activities against Escherichia coli and Staphylococcus aureus. The results suggest that silver nanoparticles dispersed in chitosan solution can be directly applied in antimicrobial fields, including antimicrobial food packaging and biomedical applications. (C) 2008 Elsevier B.V. All rights reserved.