Carcinomas produce large amounts of prostaglandin (PG) E2, which play an important role in suppression of non-specific cellular immune reaction in tumor-bearing individuals. PG synthesis inhibitor can restore the immune activity against tumors. The anti-tumor activity of indomethacin was investigated in CDF1 mice (BALB/c X DBA/2) implanted intraperitoneally with mouse colon adenocarcinoma 26 (5 X 10(5) or 2 X 10(5) cells) in a model study to prevent peritoneal recurrence after surgery for gastrointestinal cancers. Oral administration of indomethacin (0.002% water solution as drinking water) depressed and inhibited the disseminated tumor growth in the abdominal cavity, and prolonged the survival time, resulting in 30-50% cures of mice. The treatment combined with a small intraperitoneal dose of Picibanil (OK-432) (0.5 mg/kg twice weekly), which activates macrophages in the abdominal cavity, cured 90% of mice. An intraperitoneal dose of 16,16-dimethyl-PGE2 (5 micrograms/mouse, daily) reduced the anti-tumor activity of indomethacin. The results suggest that indomethacin treatment relieved the endogenous(tumor cell- and macrophage-produced) PGE2-mediated immunosuppression. It is postulated that PG-synthesis inhibitor in combination with chemotherapeutic agents, immunotherapeutic agents and low dose radiation, may provide a good therapeutic tool to prevent the development of peritoneal carcinomatosis, particularly in the cases having a small number of residual cancer cells or micrometastases in the abdominal cavity after surgery.