Antibacterial activity of the starch/chitosan blend films against E. coli B/r was evaluated by using the optical density method. In order to eliminate the influence of acids on antibacterial activity of starch/chitosan blend films, acetic acids remaining in the composite films were neutralized by washing with a 0.2 mol l−1 Na2CO3 aqueous solution followed by distilled water. 0.05 g dry films was added into 5 ml medium (Nutrient broth from Difico, Michigan, USA) including two percent E. coli B/r culture where E. coli B/r grew in a logarithmic growth phase and then incubated at 37 °C for 48 h. During the incubation, the turbidity of the medium was measured at 650 nm every 2 h. All of the operations were done in aseptic condition or by aseptic technique.