In this study, the chitosan and its derivatives were tested for their preservative activities for field natural rubber (NR) latex. A series of chitosans with different molecular weights were obtained by nitrous acid depolymerization. The chemically modified chitosans, N-carboxymethyl chitosan (NCMCh), N-sulfated chitosan (NSCh), and N-(2-hydroxy)propyl-3-trimethylammonium chitosan chloride (NHTACh), were prepared from high and low-molecular weight chitosans. Preservative activities for field NR latex of these chitosans were investigated based on the measurement of volatile fatty acids (VFA) number of the treated latex. The preservative activities of chitosan increased with decreasing molecular weights. The low-molecular weight NSCh and NHTACh exhibited good preservative activity for the latex. By the use of low-molecular weight NHTACh in combination with octylphenol poly (ethyleneglycolether) (Nonidet P40), the latex was successfully preserved for more than 1 month in the low-ammonia condition. The results showed an attractive feature to develop the preservative system, which was possible to reduce the concentrations of ammonia and carcinogenic nitrosamine in the NR latex. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 913-921, 2012