To prevent dissemination to the peritoneum following surgery for gastric cancer, examinations were performed with mouse ascites cancer models and the objective of enhancing the combined effect of MMC, CDDP and 5-FU. Pharmaceutical dosage was set at approximately 30% of the increased life span (ILS) for Meth-A. The dosage was 0.2 mg/kg for MMC, 1 mg/kg for CDDP and 37.5 mg/kg for 5-FU. Results indicated that simultaneous combination of 0.2 mg/kg of MMC and 1 mg/kg of CDDP extended life significantly more than individual administration, and clinical application of this combination was considered to be possible. In contrast, simultaneous combination of 1 mg/kg of CDDP and 37.5 mg/kg of 5-FU had no clear life-extending effect compared to individual administration of each. Some biochemical modulation of the administration method was thus considered necessary.