A new dosage formulation (5-FU-MS), composed of 5-fluorouracil (5-FU) incorporated in polyglactin microspheres, was studied by animal experiments, which revealed the following results. Intraperitoneal 5-FU-MS delivered greater concentrations of 5-FU for longer periods selectively to the intraperitoneal tissues, whereas it delivered a lower concentration to the blood plasma, than 5-FU solution did in rats. In mice, the 50% lethal dose value of intraperitoneal 5-FU-MS was 535 mg/kg. The toxicity of 5-FU-MS was less than 1/2 that of intraperitoneal 5-FU solution, of which the 50% lethal dose value was 242 mg/kg. Intraperitoneal 5-FU-MS at 200 mg of 5-FU/kg improved the survival curve of mice with peritoneal carcinomatosis better than the identical dose of 5-FU solution.