Ceruloplasmin (CP), the blue oxidase present in all vertebrates, is the major copper-containing protein of plasma. It has been proposed that oxidation of L-3,4-dihydroxyphenylalanine (DOPA) may contribute to the pathogenesis of neurodegenerative disorders. The effect of the oxidized products of DOPA on the modification of human CP was investigated. When CP was incubated with the oxidized L-DOPA, the protein was induced to be aggregated and ferroxidase activity was decreased in a time-dependent manner. Radical scavengers and catalase significantly inhibited the oxidized DOPA-mediated CP aggregation. Copper chelatrors, Diethylenetriaminepenta acetic acid (DTPA) and Diethyldithiocarbamic acid (DDC), also inhibited the oxidative modification of CP. The results suggested that DOPA oxidation led to the formation of free radical and induced the CP aggregation.