In order to provide the water-soluble and biodegradable macromolecular prodrug of 5-fluorouracil (5FU), the fixation of 5FUs to 6-O-carboxymethyl chitin(CM-chitin) through pentamethylene, monomethylene spacer groups via amide, ester bonds was carried out. The obtained CM-chitin/5FU conjugate showed the slow release of 5FU and exhibited remarkable antitumor activity against P388 lymphocytic leukemia in mice by intraperitoneal(i.p.) implantation/i.p. injection.