Polymethoxyflavones in which all hydroxyl groups are capped by methyl residues show high membrane permeability, metabolic resistance, and diverse biological activities such as cell growth inhibitory effects responsible for their anti-cancer activities. They are expected to be as biotherapeutic drugs for maintaining human health via contributing prevention and treatment of serious illnesses such as cancers, neurodegenerative diseases, inflammatory bowel diseases, lipid metabolism disorders, and so on. Human monoblastic leukemia U937 cell line has been used as an excellent in vitro cell model system for studying macrophage differentiation induced by various cell differentiation inducers such as all-trans retinoic acid (ATRA). In this study, we investigated the influences of two typical polymethoxyflavones, such as nobiletin and tangeretin, on the ATRA-induced superoxide anion (O₂^-)-generating ability of U937 cells. Nobiletin and tangeretin suppressed cell proliferation of U937 cells in a dose-dependent manner. At a concentration of 10 μM, tangeretin drastically brought about down-regulation of the ATRA-induced O₂^--generating ability (to ~15% of ATRA-treated cells) although nobiletin showed moderate inhibitory effects on the ATRA-induced O₂^--generating ability (to ~65% of ATRA-treated cells). Quantitative RT-PCR and immunoblotting revealed that tangeretin down-regulates the ATRA-induced O₂^--generating ability via suppressing gene exprerssion levels of gp91-phox (mRNA: to ~75%, protein: to ~70% of ATRA-treated cells) and p47-phox (mRNA: to ~75%, protein: to ~40% of ATRA-treated cells) genes. These findings demonstrated that tangeretin shows not only the inhibitory effects on cell growth but also the strong suppressing effects on the ATRA-induced O₂^--generating ability of U937 cells.