A new delivery formulation (5FU-MS) of 5-fluorouracil (5FU), 5FU incorporated in microspheres composed of poly(glycolide-co-lactide) matrix, has been developed for the treatment of peritoneal carcinomatosis, and is designed to slowly release the incorporated 5FU for 3 weeks. Intraperitoneal 5FU-MS distributed higher concentrations of 5FU to the intraperitoneal tissues, such as the omentum and the mesentery, for a longer period with lower blood plasma concentrations than did the aqueous 5FU solution in rats. In experiments using mice, the lethal toxicity, determined by the probit method, in 5FU-MS was reduced to less than half that in aqueous 5FU solution. We evaluated the therapeutic effects on peritoneal carcinomatosis induced by the intraperitoneal inoculation of B-16 PC melanoma cells. The therapeutic effects of 5FU-MS were enhanced when compared with both the equivalent doses and same toxicity doses of the aqueous 5FU solution.