In native superoxide dismutase, copper(II) is bound to four histidine nitrogens in a puckered square planar arrangement, the angels NCuN for the two couples of trans nitrogen atoms being 160° and 230°, respectively. One of these nitrogens belongs to an histidinate residue bridging the zinc ion [1]. The EPR spectrum shows a large rhombic component [2]. Anions like CN−, N 3−, NCS−, NCO− substitute in our opinion for a histidine nitrogen [3,4]. Zinc deprived SOD shows an axially symmetrical EPR spectrum [5] indicating that the removal of the bridging requirement leads to a less distorted chromophore. Water proton NMRD of solutions containing SOD have indicated the presence of a coordinated axial water molecule [6]. The same measurements performed on the zinc deprived derivative (Fig. 1) have shown that water is still present, although loosely bound as in the native SOD. The affinity constants of the anions N 3− and NCS− at pH 6.0 for zinc deprived SOD (160,26M− 1 closely compare with those for the native enzyme (90,120 M− 1). ▪ These data indicate that anions do not substitute for the bridging histidine in native SOD. The coordination geometry of the anion adduct is similar for the two enzyme derivatives and the major effect of zinc removal is probably that of providing a more tetragonal chromopore.