Five types of flavonol were isolated from seabuckthorn (Hippophae rhamnoides) and identified by mass, 1H- and 13C-NMR. The proliferations of human promyelotic leukemia HL-60 cells were inhibited as the concentrations of these flavonols were increased. The order of the extent of growth inhibition by the flavonols at a concentration of 20 microM is as follows: pentamethylquercetin > syringetin > isorhamnetin > quercetin > kaempherol > myricetin. Apoptotic morphological changes of the nucleus, including chromatin condensation were induced in the HL-60 cells treated with quercetin, kaempherol and myricetin, respectively, but not in the cells treated with the other flavonols. The fragmentations of DNA by quercetin, kaempherol and myricetin, respectively, to oligonucleosomal-sized fragments, a characteristic of apoptosis, were observed to be dose-dependent in the HL-60 cells. These findings suggest that growth inhibition by quercetin, kaempherol and myricetin, respectively, results from the induction of apoptosis by these flavonols. The other flavonols (pentamethylquercetin, syringetin and isorhamnetin) having methoxy (-OCH3) group inhibited more strongly than the above 3 flavonols without induction of apoptosis in the HL-60 cells. These findings suggest that mechanisms of growth inhibition by pentamethylquercetin, syringetin and isorhamnetin are different from the apoptosis caused by quercetin, kaempherol and myricetin.