PURPOSE: To obtain a reactive chitosan derivative, by coagulating an acid solution of chitosan by contacting it with an alkali solution and reacting the coagulated product with an acylating agent.

CONSTITUTION: Chitesan is dissolved in an acid. The resulting solution is contacted with an alkali solution to form a coagulated product in the fiber or film form. Then the coagulated product thus produced is dispersed in an inert solvent and reacted with an acylating agent. As the acylating agent, an anhydride or chlorid a of a carboxylic acidmis advantageously used. Exemplary of these compounds are acetic anhydride monochloroacetic anhydride, acrylic anhydride and methacrylic anhydride. As the inert solvents, mention can be made of aprotic solvents such as amide compounds, aromatic hydrocarbon compounds, aliphatic chlorine compounds and aliphatic ether compounds, the these solvents can be used in combination with an alcohol.