Аннотация:Horseradish peroxidase /HRP/ is known as a catalyst for the oxidation of some organic compounds by hydrogen peroxide and oxygen /1,3/. The mechanism of HRP oxidase activity has been proposed /2/ to include superoxide radical C2- coordinated on Fe/III/HRP. Recently we have found /4/ the oxidase activity of HRP in the oxidation of bilirubin /BR/ bound to the human serum albumin /HSA/ but not to the BR. The aim of this paper is the study of the radical intermediates and the effect of HSA by ESR spectroscopy.