The property of serum and partially purified ceruloplasmin (50%) to oxidize N,N-dimethyl-p- o phenylenediamine is impaired by exposure to UV radiation at 2537 A. When the relative enzymatic activity of samples subjected to UV radiation is plotted against the radiation dose, the relationship was found to be exponential (A = Aoe-kD). The loss of enzymatic activity was correlated with an increase in direct reading copper of the samples. Furthermore, several common -SH inhibitors had no effect on the enzymatic activity. Therefore, it is postulated that the mechanism of inactivation of ceruloplasmin oxidative activity by UV radiation is due to a change in state of the bound copper to "unbound" copper in the protein molecule.