H-1 and N-14 electron nuclear double resonance (ENDOR) spectra have been obtained from native and specifically deuterated randomly-oriented frozen-solution specimens of ((Cu2Zn2)-Zn-IIII)-superoxide dismutase from bovine and human erythrocytes at about 5 K. Using powder simulation routines combined with specific deuteration, 11 out of the 15 protons in the vicinity of the Cu ion could be assigned to specific ENDOR lines and associated with proton positions in the four coordinated histidine rings (His 44, 46, 61, 118 in the enumeration of the bovine isoenzyme) or with the beta-protons of C beta of His 44. Nine of these were assigned on the basis of both simulation and the response to specific deuteration, two by simulations only. The water protons remained unassigned. The N-14 ENDOR. resonances were analyzed in terms of two groups of nitrogen interactions comprising two equivalent couplings in each group. One group, probably associated with His 44 and His 46, has coupling values close to those found recently for copper tetraimidazole (Scholl, H.-J.; Huttermann, J. J. Phys. Chem. 1992, 96, 9684). The other group, comprising His 61 and His 118, has the same coupling along g(max) but an "in-plane" interaction larger by about 16%. The combined assignment of the proton and nitrogen ENDOR response has allowed the reconstruction of the prosthetic group of the copper site in SOD and the confirmation that it is unchanged from that obtained by X-ray analysis at room temperature.