Bioassay-guided fractionation of the extracts of Zieridium pseudobtusifolium and Acronychia porteri led to the isolation of 5,3'-dihydroxy-3,6,7,8,4'-pentamethoxyflavone [1], which showed activity against (KB) human nasopharyngeal carcinoma cells (IC50 0.04 micrograms/ml) and inhibited tubulin assembly into microtubules (IC50 12 microM). Two other known flavonols, digicitrin [2] and 5-hydroxy-3,6,7,8,3',4'-hexamethoxyflavone [5], were also isolated together with three new ones, 3-O-demethyldigicitrin [3], 3,5,3'-trihydroxy-6,7,8,4'-tetramethoxyflavone [4], and 3,5-dihydroxy-6,7,8,3',4'-pentamethoxyflavone [6]. All of these flavonols showed cytotoxic activity against KB cells.