The 1:1 complex of copper (II) and human serum albumin (HSA) slowly reacts with radiolytically generated *O2- radical-anion at a rate constant of 6.1 x 10(6) M(-1) s(-1). Absorbance and fluorescence spectroscopies demonstrate that addition of an equimolar portion of quercetin (QH2) to the solution of the copper (II)-HSA complex induces a relocalization of the copper resulting in a ternary copper (II)-QH2-HSA complex. This form of quercetin slowly oxidizes in air-saturated solutions. A 10-fold excess urate, a plasma antioxidant, cannot displace copper (II) bound to HSA. In N2O-saturated solutions the ternary complex form of QH2 can repair the urate radical with a rate constant of 2.7 x 10(6) M(-1) s(-1) by an electron transfer reaction similar to that observed in the absence of copper (II). In O2-saturated solutions and in the absence of copper, HSA-bound QH2 fails to repair the urate radical because of the fast competitive reaction of *O2- with urate radicals. However, addition of equimolar copper (II) restores the electron transfer from QH2 to the urate radical. These contrasting results are tentatively explained either by an enhanced reactivity of copper (II) with *O2- in the ternary complex or by direct production of quercetin radicals via a copper-catalyzed reduction of the *O2- radicals by QH2.