Abstract: The characters cf powder bilirubin free radicals under UV irradiation were studied by ESR method. It has been discovered that UV light only induced an increase in the concentration of bilirubin superoxide radical and semiquinone radical, but no new free radical forms are produced. It is suggested that the free radical medium[BR⁺ -O₂] of Foote's mechanism for photoxidation of bilirubin is really composed of both of the free radicals mentioned above with simulation of bilirubin ESR spectrum, a further mechanism about the formation of bilirubin free radical under UV irradiction was given. And the relationship between bilirubin photochemical characters and the both free radical concentrations as well as their activity in some conditions was also deduced.

Key words: Bilirubin free radical, ESR, UV