In order to provide a polymeric drug of 5-fluorouracil (5FU) with reduced side-effects, having affinity for tumor cells and exhibiting high antitumor activity, four kinds of chitosan derivatives were synthesized carrying 5FUs through some kinds of spacers via ether, amide, ester, or carbamoyl bonds. The release behaviors of 5FU and 5FU residues were studied in vitro at 37 "C in various aqueous media. The antitumor activity was tested against p-388 lymphocytic leukemia in female CDF, mice in vivo by intraperitoneal injection of the polymeric drugs followed by intraperitoneal injection of the leukemia cells (i. p./i.p.). Chitosan fixing 5FU at 2-position through a hexamethylene spacer and carbamoyl linkages (9) exhibits a higher effect with respect to prolongation of life than free 5FU. The chitosan-5FU conjugates obtained do not display acute toxicity in the high dose range.