Abstract: Oxymatrine (OMT) was isolated from Sophora flavescens Alt. Studies showed that the content of hepatic microsomal cytochrome P-450 (P-450) was significantly increased by OMT (100mg/kg). Increased antitumor effect and content of P-450 were found in Ehrlich carcinoma-bearing mice with OMT (100mg/kg) plus cyclophosphamide (CPA) (60 mg/kg). The synergism of antitumor effect was due to increased level of alkylating metabolites of CPA by OMT. Furthermore, antitumor effect of OMT (100mg/kg) plus CPA (30mg/kg) was similar to that of CPA (60mg/kg) but inhibitory effect of CPA on WBC was reduced.