A new type of cell growth inhibitory substance was isolated and purified from a powder of the root of Panax ginseng C.A. Meyer, which has been commonly used for the treatment of various disease as a commercial medical drug by the name of Korean Red Ginseng Powder. Data from the infrared spectra, proton and carbon-13 nuclear magnetic resonance, and high resolution mass spectra were identical with those of authentic panaxytriol. Cell growth inhibition by panaxytriol seemed to be tumor-specific. Its action was supposed to be more dose-dependent than time-dependent. In vivo antitumor efficacy of panaxytriol was tested against C57BL/6 mice transplanted with B 16 melanomas. Panaxytriol (40 mg/kg) administered intramuscularly produced significant tumor growth delays in mice (p less than 0.01).