The aim of the study was establishment of mechanisms of bilirubin oxidation and their involvement in the physiological and pathological processes in the living body (EPR study of photoradiated bilirubin). The photosensitized formation of free radical of bilirubin with g=2.003 and DeltaH=1.0 mTl, under action of the blue light with lambda(max)=450 nm by means of electronic spin resonance (ESR) was shown. Irradiation of sample in vacuum by visible light does not cause formation of free radicals. Irradiation of powder of bilirubin and also of its solution in chloroform leads to formation of the radical of bilirubin. The analysis of a spectrum (ESR) as powder also its solution in chloroform, that induced free radical belongs to bilirubin but not of solution was shown. Irradiation of a solution of bilirubin in chloroform causes absorption spectrum with lambda(max)=650 nm, characterized for absorption of solutions biliverdin in chloroform.