Ceruloplasmin oxidation by hypochlorite results in the bleaching of the protein solution; the oxidase activity remains, however, unchanged. Hypochlorite exerts a complex effect on the protein activity. After a short-term (5 min) incubation with hypochlorite the enzyme activity increases with a further progressive decrease. It is supposed that the oxidase activity of ceruloplasmin is not coupled with copper ions of the first type responsible for the blue staining of the protein solution. The low susceptibility of functional properties of ceruloplasmin to hypochlorite raises its potency as an antioxidative agent.