Studies were made of the effects of the embolization and drug releasing of 1-hexylcarbamoyl-5-fluorouracil (HCFU), which has been microencapsulated with alginic acid (HCFUmc), on an unresectable tumor. HCFUmc showed nearly spherical in a uniform size of about 400 micron in diameter. In-vitro elution tests revealed that the gradually-releasing effect of the elution of HCFU fraction continued for over 120 hours. This effect was examined using mongrel adult dogs, on whom hepatic arterial embolization was surgically performed by the use of microcapsules, with favorable results. The anti-tumor activity of HCFUmc was evaluated by the intraperitoneal transplantation of ascites sarcoma 180 into ddY mice, with a significantly higher effect obtained than that of HCFU-lipiodol (HCFULIP). Intra-hepatic injections of HCFUmc and HCFULIP were surgically conducted for comparison on two groups of mongrel adult dogs, respectively. With the HCFUmc-injected group, the HCFU fraction concentration stayed at significantly lower values during the first 24 hours, whereas the concentrations of HCFU fraction and 5-FU determined at the end of the second 24 hours indicated significantly higher values. HCFUmc, showing favorable gradual-releasing behavior, is considered a promising chemoembolization agent as well as expected to prove a new anti-tumor agent for peritoneal carcinomatosis treatment.