The reaction of human ceruloplasmin and anion treated ceruloplasmin with diethyldithiocarbamate was studied at pH 5.5. The analysis of optical and EPR spectra at 9 GHz showed that ceruloplasmin contains five paramagnetic copper ions, two of which, X and Y, not involved in enzymatic activity, are chelated by diethyldithiocarbamate; the complex thus formed is easily removed by high-speed centrifugation. However, the enzyme depleted of these two X and Y copper ions is able to compete with the Cu(II)-diethyldithiocarbamate complex, as time elapses, recovering both Cu(II) atoms. In addition diethyldithiocarbamate acts as a reducing agent for the two type-I copper atoms when added in large excess to the enzyme or the anion treated enzyme.