Copper incorporation into ceruloplasmin during ceruloplasmin synthesis was studied by comparing LEC and control rats. Major 132 and 136 kDa ceruloplasmins were found in microsomes and the Golgi apparatus, respectively, isolated from liver homogenates of LEC and control rats. Copper analysis showed that no copper was detected in the ceruloplasmin in the microsomes of either rat. Copper was present in ceruloplasmin in the Golgi apparatus and serum of controls, while it was not detected in ceruloplasmin in the Golgi apparatus and serum of the LEC rat. These results indicate that copper is incorporated into ceruloplasmin in the Golgi apparatus of normal hepatocytes. LEC rats fail to incorporate copper into ceruloplasmin in the Golgi apparatus.