1. We have developed a procedure for preparing derivatives of bovine superoxide dismutase in which primarily the Cu binding sites are occupied by Cu2+ (2 Cu2+-) and in which both the Zn and Cu binding sites are occupied by Cu2+ (4 Cu2+-). 2. The 2 Cu2+ protein shows approximately one-half the superoxide dismutase activity of an equivalent amount of native protein. A two-fold enhancement of the activity of 2 Cu2+-dismutase was observed upon occupation of the Zn sites either with Zn2+ or Cu2+. 3. The electron paramagnetic resonance spectrum of 4 Cu2+ protein was recorded over the temperature range 5-100 degrees K and the results suggest an antiferro-magnetic interaction between Cu2+ in the Zn site and Cu2+ in the Cu site having a coupling constant of approx. 52 cm-1. 4. The binuclear Cu2+ complex was found to accept only one electron from ferrocyanide. 5. One-half the total Cu+ of dithionite reduced 4 Cu+ protein was found to react rapidly with bathocupreine sulfonate whereas the other half reacted slowly. Reduced native protein did not react with bathocupreine sulfonate below 70 degrees C.