Chitosan is known to inhibit microorganisms of concern to plants, animals, and humans. However, the effect of chitosan on human enteric viruses of public health concern has not been extensively investigated. The purpose of this study was to determine the effect of chitosan on three human enteric viral surrogates: murine norovirus 1 (MNV-1), feline calicivirus F-9 (FCV-F9), and (ssRNA) bacteriophage MS2 (MS2). Chitosan oligosaccharide lactate (molecular weight of 5,000) and water-soluble chitosan (molecular weight of 53,000) at concentrations of 1.4, 0.7, and 0.35% were incubated at 37 degrees C for 3 h with equal volumes of each virus at high (approximately 7 log PFU/ml) and low (approximately 5 log PFU/ml) titers. Chitosan effects on each treated virus were evaluated with standardized plaque assays in comparison to untreated virus controls. The water-soluble chitosan at 0.7% decreased the FCV-F9 titer by approximately 2.83 log PFU/ml, with decreasing effects at lower concentrations, and also decreased MS2 at high titers by approximately 1.18 to 1.41 log PFU/ml, regardless of the concentration used. Chitosan treatments at the concentrations studied had no effect on MNV-1 at high titers. Chitosan oligosaccharide showed similar trends against the viruses, but to a lesser extent compared with that of water-soluble chitosan. When lower virus titers (approximately 5 log PFU/ml) were used, plaque reduction was observed for FCV-F9 and MS2, but not MNV-1. The use of higher-molecular-weight chitosan and at higher concentrations with longer incubation may be necessary to inactivate MNV-1. These results in the plaque reduction of human enteric virus surrogates by chitosan treatment show promise for its potential application in the food environment.