Twenty-nine flavonoids and six hydrolyzable tannins were studied for their inhibitory activity against human immunodeficiency virus (HIV)-1 protease using fluorescence and HPLC assays. Among the flavonoids, flavones, flavanones, flavonols, catechols and chalcones, the flavonols were the most active category while flavanones and catechols displayed low activity. Quercetin was the most potent inhibitor of the target enzyme with an IC₅₀ value of 58.8 μM, while butein and luteolin showed moderate activity. Of the hydrolyzable tannins tested, three ellagitannins which contain a hexahydroxydiphenoyl (HHDP) unit linked to the O-3 and O-6 positions of the sugar, were found to strongly inhibit HIV-1 protease. The IC₅₀ values of corilagin and repandusinic acid on HIV-1 protease were 20.7 and 12.5 μM, respectively.