The iron(III) and Cu(II) reducing ability of common antioxidants may be an indirect measure of their prooxidant activity, because the reduced ions may generate reactive oxygen species (ROS). This study aimed to develop a new Fe(III)-ferrozine spectrophotometric prooxidant activity assay using a protein - based solid biosensor prepared from Ca(II) and chicken egg whites. This new assay involved the reduction of Fe(III) ions to Fe(II) by antioxidant compounds and the binding of the formed Fe(II) to the solid biosensor. As an indicator of the prooxidant activity of antioxidants on proteins, the protein - bound Fe(II) was colorimetrically determined with ferrozine at 562 nm. The prooxidant activities of antioxidant compounds such as gallic acid, catechin, epicatechin, epigallocatechin gallate, and chlorogenic acid were determined across a wide range of final concentrations between 0.25 and 2500 mu M. Most of the tested compounds showed prooxidant activity above a concentration of 2.50 mu M with the Fe(III)-based method. In addition, the findings of the new method were compared with those of the modified Cu(II)-neocuproine (Nc) method previously developed by the same authors based on the principle of copper(II) reduction by antioxidants in the presence of a Ca(II)-proteinate sensor.