Copper uptake from human ceruloplasmin (Cp) into cells of a human erythroleukemic cell line, K562, was investigated. The interaction between ascorbic acid and the copper atoms in ceruloplasmin was a focal point of the study. Nondenatured 67Cu-labeled ceruloplasmin (67Cu-Cp) was prepared by an ascorbate-catalyzed exchange of Cp with 67CuCl2 in vitro. The complex was stable, even in the presence of 1.0 mM ascorbate. Adding K562 cells and incubating at 37 degrees C resulted in an immediate transfer of 67Cu from ceruloplasmin to the cells. At 37 degrees C the copper accumulated by the K562 cells resisted dissociation by mild acid washing. The rate of transfer of 67Cu was proportional to the Cp concentration in the medium. Ascorbate (100 microM) enhanced the uptake of 67Cu at least fourfold. D-Isoascorbate worked as well as L-ascorbate, suggesting that the reducing potential of the vitamin (or its isomer) was important in the uptake of copper. Approximately 20% of the 67Cu absorbed into the cytosol was precipitable with antibodies to Cu-Zn superoxide dismutase (Cu-Zn SOD). Ascorbate, however, did not enhance the incorporation of radioactivity into Cu-Zn SOD, suggesting that copper may not be the only rate-limiting factor in the synthesis of this enzyme in K562 cells. The possible relevance of these observations to vitamin C deficiency is discussed.