Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)The antimicrobial potential of alginate films (AFs) and alginate/chitosan composite films with two different mass proportions of the biopolymers, 82.5:17.5 (CF1) and 65:35 (CF2), containing potassium sorbate (KS) or natamycin was evaluated. At the practical limit of KS addition (0.17 g KS per gram of alginate) for pure AFs, no inhibition zones were observed against Debaromyces hansenii, Penicillium commune and Penicillium roqueforti by the agar diffusion test. Above this concentration, films became opaque, brittle and showed a whitish precipitate over their surface, making them not suitable for use. However, alginate and alginate/chitosan composite films containing natamycin were able to inhibit the growth of the three microorganisms listed above. Natamycin was effective at concentrations as low as 0.005 g per gram of biopolymer for AF and 0.01 g per gram of biopolymer for both composite films, alginate/chitosan 65:35 (CF1) and alginate/chitosan 82.5:17.5 (CF2) against all microorganisms tested. The inhibitory zone diameter increased as concentration of natamycin increased. Active films containing 0.04 g natamycin per gram of biopolymer obtained in the present study exhibited suitable functional attributes and showed excellent perspectives as active antimicrobial films intended for food protection applications. Copyright (c) 2012 John Wiley & Sons, Ltd.268479492Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPESP [2005/55499-3]