The antioxidant activity of several plant catechol derivatives was tested in buffer, plasma, and human erythrocytes. In buffer, chlorogenic acid (CGA), caffeic acid (CA), and dihydrocaffeic acid (DCA) reduced ferric iron equally well in the ferric reducing antioxidant power (FRAP) assay. Low concentrations of the polyphenols enhanced the ability of plasma to reduce ferric iron by about 10%. In plasma, lipid hydroperoxide and F2-isoprostane formation induced by a water-soluble free radical initiator were reduced by CGA at concentrations as low as 20 microM. During incubation at 37 degrees C, human erythrocytes took up DCA, but not CGA, and intracellular DCA enhanced the ability of erythrocytes to reduce extracellular ferricyanide. When intact erythrocytes were exposed to oxidant stress generated by liposomes containing small amounts of lipid hydroperoxides, extracellular CGA at a concentration of 5 microM decreased both lipid peroxidation in the liposomes, and spared alpha-tocopherol in erythrocyte membranes. These results suggest that the catechol structure of these compounds convey the antioxidant effect in plasma and in erythrocytes.