Ag-Cu and Ag-Co proteins were prepared in which Ag+ resides in the native Cu site of [bovine erythrocyte] superoxide dismutase and either Cu2+ or Co2+ reside in the Zn site. EPR spectrum of the Cu and the visible absorption spectrum of the Co greatly resembled those of either Cu4 or Cu2,Co2 proteins, respectively, in which the Cu of the native Cu sites has been reduced. Unlike cyanide, azide anion would not perturb the EPR spectrum of Ag2,Cu2 protein. Since azide produces the same perturbation upon the EPR spectrum of native and Cu2 proteins, it must bind to the Cu and not the Zn of superoxide dismutase. A model of the metal sites of the enzyme was fitted to a 3-.ANG. electron-density map using an interactive molecular graphics display. The model shows that histidine-61, which appears to bind both Cu and Zn, does not lie in the plane of the Cu and its 3 other histidine ligands, but occupies a position intermediate between planar and axial. This feature probably accounts for the rhombicity of the EPR spectrum and the activity of the enzyme.