Individual preparations of CP isolated from one normal donor and two homozygotes for the “Wilson's disease gene” were subjected to a comparative structural and analytical examination. Analysis of CP tryptic hydrolysate by the peptide map technique revealed 64–66 peptides in each sample. Patient P.'s peptide map lacked one acidic, one neutral, and two basic peptides, but had one basic peptide not found in the normal control. Patient R. lacked one acidic and one basic peptide present in the normal. These changes in the peptides of both patients did not affect the capacity of the protein to bind copper but were associated with diminished oxidase activity.