Oxidative stresses are involved in the process of chronic inflammatory diseases. The objective of this study was to evaluate and compare the anti-inflammatory activity of major dietary antioxidants. Murine RAW264.7 macrophages (4 x 10(6) cells) were incubated in DMEM containing 10% FBS supplemented with 5 to 20 microM of alpha-tocopherol, beta-carotene or quercetin for 24 hrs, and then treated with LPS for 16 hrs. The medium was collected to measure TNF-alpha, IL-6, PGE2 and nitrite. Expressions of cyclooxygenase-2(COX-2) and inducible nitric oxide synthase(iNOS) were determined using the harvested cells. Results indicate that TNF-alpha and IL-6 accumulations were significantly reduced by 5 to 20 microM quercetin treatment, and 20 microM of alpha-tocopherol treatment. Nitrite release was significantly reduced by 5 microM quercetin treatment. However, PGE2 accumulation was not affected by any of the antioxidants used. Expressions of COX-2 and iNOS were effectively reduced by 5 microM quercetin treatment. These findings indicate that dietary antioxidants possess significant anti-inflammatory activities, and quercetin is the most potent antioxidant.