We investigated the antioxidant and antiinflammatory activities of a flavonoid-rich polyphenolic fraction of cocoa. Cocoa polyphenol (CP) was fractionated from commercial cocoa powder and contained 468 mg/g of gallic acid-equivalent phenolics and 413 mg/g epicatechin-equivalent flavonoids. CP exhibited a dose-dependent free radical-scavenging activity as determined by both 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) and 2,2'-diphenyl-1-picrylhydrazyl radical scavenging assays. CP also dose-dependently inhibited xanthine oxidase activity and 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced superoxide-anion generation in cultured human promyeolcytic leukemia HL-60 cells. Oral administering of CP (4, 20, 40, and 200 mg/kg body weight) to ICR mice 1 h prior to TPA (10 nmol) inhibited ear edema at 5 h in a dose-dependent manner. The levels of COX-2 expression induced in mouse skin after 4-h treatment with topical TPA (10 nmol) was also diminished significantly by pretreating CP (40 or 200 mg/kg) for 30 min. CP at the same doses inhibited TPA-induced nuclear translocation of p65 and subsequent DNA binding of NF-kappaB at 1 h by blocking the degradation of IkappaBalpha in mouse skin. Moreover, phosphorylation of p38 mitogen-activated protein kinase in ICR mouse skin, measured 4 h after TPA treatment, was suppressed by oral pretreatment of CP (40 or 200 mg/kg). Although extracellular signal-regulated protein kinase 1/2 phosphorylation was unaffected, CP inhibited the catalytic activity of extracellular signal-regulated protein kinase 1/2 in TPA-stimulated mouse skin. Since cellular proinflammatory and prooxidant states are closely linked to tumor promotion, the antioxidant and antiinflammatory properties of CP may constitute the basis of possible antitumor promoting effects of this phytochemical.