The antitumor effects of Xiao-Chai-Hu-Tang (Shosaiko-To: SHX) with or without 5-fluorouracil (5-FU) or cyclophosphamide (CY) were examined in an experimental system of lung metastasis induced by Lewis lung carcinoma in C57BL/6crSlc mice. Lewis lung carcinoma cells were implanted into the footpads of the mice. Ten days later, the implanted tumors were surgically removed. The effects of SHX were evaluated by the number of lung surface nodules present 14 days after removal of the implanted tumor. The administration of SHX, p.o. (300 mg/kg×2/day ×10) caused the antimetastatic effect. Therapy with SHX plus 5-FU or CY significantly inhibited the development of lung metastases. The number of peritoneal macrophages and the degree of the binding of C3 cleavage products (C3b) to macrophages were enhanced in the mice treated with SHX. Lung metastases were inhibited by the i.v. administration of peritoneal macrophages activated with SHX, p.o. These findings raise the possibility that SHX may have clinical value in the prevention of cancer metastasis.