Flavonoids have been proposed to have anti-inflammatory activity. Reactive oxygen species (ROS) are important mediators in the inflammatory response. We examined the effects of luteolin, quercetin and eriodictyol on ROS production in phorbol-12-myristate-13-acetate (PMA)-stimulated U937 cells. Eriodictyol inhibited ROS production at a lower inhibition concentration 50% (IC50) then luteolin and quercetin (eriodictyol : 0.822 µM, quercetin : 1.134 µM, luteolin : 1.026 µM). Moreover, eriodictyol inhibited ROS production in a dose- and time-dependent manner. Moreover, the p47phox subunit of nicotinamide adenine dinucleotide phosphate oxidase regulated PMA-mediated ROS production. After eriodictyol treatment, p47phox clustering and translocation also decreased on stimulation of U937 cells with PMA. This study suggests that eriodictyol governs immune responses by modulating p47phox clustering and translocation and ROS production in monocytes – evidence that eriodictyol is an immune regulator of ROS production during inflammatory responses.