A cytotoxic sesquiterpene against L1210 cell has been isolated from the root of Curcuma domestica. Its structure was identified as ${eta}-sesquiphellandrene$. The cytotoxicity-potentiating substance was (+)-ar-turmerone. (+)-ar-Turmerone potentiated the cytotoxicity of ${eta}-sesquiphellandrene$(5 fold in $ED_{50}$ value) and an unknown sesquiterpene which was isolated from the root as well, and that of aurapten(6.3 fold) isolated from the unripe fruit of Poncirus trifoliata. Moreover, it potentiated the cytotoxic activities of MeCCNU 10 fold and cyclophosphamide 10 fold. Except the fact that all the effective cytotoxic substances possess relatively good lipophilicity, no relationship between structures of the cytotoxic substances and the cytotoxicity-enhancing effect of (+)-ar-turmerone could be observed.