The effects of 17 flavonoids, isolated from two flavonoid-rich Scutellaria species (Scutellaria baicalensis Georgi and Scutellaria rivularis Wall) used in traditional Chinese medicine, on HL-60 cells were assessed by WST-8. Ten of the flavonoids inhibited the proliferation of HL-60, as shown by IC50 values used as indexes of the inhibition. 2',3',5,7-tetrahydroxy flavone (IC50=9.5 microM), apigenin (15.0 microM), viscidulin III (17.4 microM), wogonin (17.4 microM) and luteolin (18.4 microM) were more effective than baicalein (23.0 microM) which reportedly inhibits the proliferation of some cancer cell lines. Others were less effective, and oroxylin A stimulated the proliferation. Scutellaria rivularis, used for the treatment of tumors in the clinic, contained flavonoids that were more inhibitive than those in Scutellaria baicalensis. These results are demonstrative of some reasons for the use of Scutellaria rivularis as a crude antitumor drug.