In a previous study, we found that human ceruloplasmin (hCP) promotes iron uptake rather than release in BT325 cells, a human glioma cell line. In this study, we investigated the effect of ferroxidase activity of hCP and different species of ceruloplasmins on CP-mediated iron uptake by the cells. The cells were incubated for 30 min at 37 degrees C with 1 microM 59Fe2+ with or without 150 microg/ml of the untreated and the ferroxidase-defective hCPs (apohCP and heat-inactivated hCP) or different species of ceruloplasmins (human CP, rabbit CP and bovine CP). The untreated hCP induced a significant increase in iron uptake by BT325 cells, while ferroxidase-defective hCPs with (heat-inactivated hCP) or without cooper ions (apohCP) had no such role. The untreated hCP increases significantly internalized iron but not membrane-bound iron, implying that hCP stimulated iron entry into the cell rather than increased extracellular binding of iron to the cell surface. All species of ceruloplasmins could promote iron uptake by the cells and the difference in degree of stimulatory effect among them was insignificant. These results suggested that ferroxidase activity of hCP is essential for the hCP-mediated iron uptake process and also that CP-stimulated iron uptake is not associated with copper ions in the protein, and that the effect of ceruloplasmin on iron uptake by BT325 cells is not species specific.