A new drug dosage form comprising activated carbon particles absorbing mitomycin C (MMC-CH) has the advantage of being retained in the peritoneal cavity and distributes a larger amount of mitomycin C for a long time. Some 113 patients with gastric cancer and serosal infiltration were randomly assigned intraperitoneal treatment with MMC-CH and no anti-cancer drugs. There were significant differences in survival rate between the patients who have no visible peritoneal dissemination with MMC-CH and without. Thus, intraoperative intraperitoneal treatment with MMC-CH improves survival after gastrectomy for gastric cancer and seems effective against peritoneal recurrence.