An aqueous emulsion of chitosan nanoparticles encapsulating silver oxide is prepared from silver nitrate and chitosan. The nanoparticles are positively charged with an average diameter of 300 nm. The dried particle has a spherical shape with a 100 nm diameter. The emulsion is applied onto cotton and delivers a durable antibacterial activity against S. aureus and E. coli, after 20 washings. The coefficient of friction of the treated fabric is similar to that of the untreated cotton fabric. (C) 2007 Wiley Periodicals, Inc.