n - halochitosans which are terpolymers containing 1 to 35 % of 2 - deoxy - 2 - acetaminodoglucose monomeric units , 1 to 90 % of 2 - deoxy - 2 - haloaminoglucose monomeric units , and 8 - 98 % of 2 - deoxy - 2 - haloaminoglucose monomeric units wherein the halogen is chlorine or bromine and their preparation is disclosed . the preferred preparation is by treatment of an aqueous solution of a chitosan salt with a halogenating agent .