A cytotoxic constituent was isolated by bioassay-guided procedure from the roots ofSophora flavescens Aiton (Leguminosae). The constituent was identified as sophoraflavanone G (I) by means of chemical methods and in comparsion with spectral data of standard compound. The ED(50) values of constituent I were 0.78, 1.57, 2.14 and 8.59 mug/ml against A549, HeLa, K562 and L1210 cell lines respectively. ConstituentI exhibited highly cytotoxic activities against A 549, K562 and HeLa cells, but showed a mild activity (ED(50) value, 5 mug/ml) against L1210 cells. Among the tested cell lines, A549 cells were the most sensitive to constituentI.