Cellular death by apoptosis is an active process, depending on gene transcription and protein synthesis. It was reported that nitric oxide can induce apoptosis in several cancer cell-lines. We studied effects of Phytolacca esculentum van Houtt (Phytolaccaceae) Radix water extract (PRE) on the proliferation of transplanted-L1210 cells in mice. When PRE (500 mg/kg) was administered orally once a day for 7 days after transplantation of L1210 cells to mice, DNA fragmentation of transplanted-L1210 cells was induced and mitochondrial transmembrane potential of those cells was reduced. Additionally, DNA fragmentation of L1210 cells was induced by the treatment of PRE in vitro. Also, DNA fragmentation of L1210 cells was enhanced by co-culture with the peritoneal macrophages obtained from PRE-administered mice and was partly inhibited by L-NMMA in vitro. PRE enhanced the production of nitric oxide and tumor necrosis factor-α from peritoneal rnacrophages. These results suggest that PRE induces apoptosis of transplanted-L1210 cells via directive action on L1210 cells and stimulation of nitric oxide and tumor neaosis factor-α from macrophages.