A triterpene was isolated as a cytotoxic principle from the dichloromethane extract of Korean mistletoe (KM； Viscum album colora turn) by repeated silica gel chromatography and recrystalli-zation. In in vitro analysis of cytotoxic activity using various human and murine tumor cell lines, the dichloromethane extract of KM was highly cytotoxic against these cells. We isolated the most active compound, referred to VD-3, from the dichloromethane extract of KM. The VD-3 was shown to be less cytotoxic to normal cells (murine splenocytes). From the identification of the chemical structure of VD-3 by spectral data and chemical synthesis, the compound was proven to be epi-oleanolic acid. Tumor cells treated with VD-3 showed a typical pattern of apo-ptotic cell death, such as apparent morphological changes and DNA fragmentation. These results indicate that epi-oleanolic acid is an important compound responsible for antitumor activity of KM.