An antitumor-active substance was obtained from the residue of the ethyl acetate extract of red ginseng, a traditional Chinese medicine, by chromatography on a silica gel column. From the proton and carbon-13 nuclear magnetic resonance spectra, it was identified as heptadeca-1-ene-4,6-diyne-3,9,10-triol (panaxytriol). The panaxytriol contents of red ginseng and white ginseng, determined by gas chromatography after solvent extraction and formation of trimethylsilyl derivatives, were 0.38 and 0.25 mg/g, respectively. Panaxytriol showed a growth-inhibitory activity against several tumor cell lines.