Plasma of a healthy human adult individual contains about 115 µg of copper per 100 ml. About 95% of this copper is bound to a characteristic blue protein called ceruloplasmin (Cp). Cp contains amino acids, prosthetic carbohydrate, and copper. Cp typically contains six to seven copper atoms. The catalytic activity of Cp has several possible physiological effects. One of them is that iron(II) is converted to iron(III) preliminary to its incorporation into transferrin. Cp seems, to be a molecule where some copper atoms have been lost and others have changed their function in course of evolution. The crystals of native Cp were examined by Magdoff-Fairchild et al.60 and later by Zaytsev et al. Zaytsev et al. has calculated rotation functions for the crystals of native Cp. Although Cp is relatively small, some details about the structure have been derived with the aid of electron microscopy.