The three-dimensional X-ray structure of a recombinant human mitochondrial manganese superoxide dismutase (MnSOD) (chain length 198 residues) was determined by the method of molecular replacement using the related structure of MnSOD from Thermus thermophilus as a search model. This tetrameric human MnSOD crystallizes in space group P2(1)2(1)2 with a dimer in the asymmetric unit (Wagner, U.G., Werber, M.M., Beck, Y., Hartman, J.R., Frolow, F., & Sussman, J.L., 1989, J. Mol. Biol. 206, 787-788). Refinement of the protein structure (3,148 atoms with Mn and no solvents), with restraints maintaining noncrystallographic symmetry, converged at an R-factor of 0.207 using all data from 8.0 to 3.2 A resolution and group thermal parameters. The monomer-monomer interactions typical of bacterial Fe- and Mn-containing SODs are retained in the human enzyme, but the dimer-dimer interactions that form the tetramer are very different from those found in the structure of MnSOD from T. thermophilus. In human MnSOD one of the dimers is rotated by 84 degrees relative to its equivalent in the thermophile enzyme. As a result the monomers are arranged in an approximately tetrahedral array, the dimer-dimer packing is more intimate than observed in the bacterial MnSOD from T. thermophilus, and the dimers interdigitate. The metal-ligand interactions, determined by refinement and verified by computation of omit maps, are identical to those observed in T. thermophilus MnSOD.