The antineoplastic effects of combinations of anticancer drugs (5-fluorouracil, irinotecan and cisplatin) and triterpenes (ursolic acid, betulinic acid, oleanolic acid and a Japanese apricot extract (JAE) containing triterpenes) on esophageal squamous carcinoma cells were examined by the WST-8 (2-(2-methoxy- 4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium, monosodium salt) assay in vitro and by an animal model in vivo. Triterpenes and JAE showed additive and synergistic cytotoxic effects, respectively, on esophageal squamous carcinoma cells (YES-2 cells) by combinational use of 5-fluorouracil. JAE and 5-fluorouracil induced cell cycle arrest at G2/M phase and at S phase, respectively, and caused apoptosis in YES-2 cells. A new animal model of esophageal cancer causing tumor colonization of the peritoneal cavity and producing bloody ascites was made by injecting YES-2 cells into the peritoneal cavity of a severe combined immunodeficiency mouse. In this model, 5-fluorouracil inhibited colonization of tumor cells in the peritoneum. The addition of JAE to 5-fluorouracil augmented the suppression of experimental metastasis of the peritoneum. The numbers of peritoneal nodules of more than 2 mm in diameter in mice treated with 5-fluorouracil and JAE were less than those in mice treated with 5-fluorouracil alone or JAE alone. These results suggest that triterpenes, especially JAE, are effective supplements for enhancing the chemotherapeutic effect of 5-fluorouracil on esophageal cancer.