The principal functional subunit of the approximately 3500 kDa extracellular Lumbricus terrestris hemoglobin is a 213 kDa dodecamer of four chemically distinct globin chains, consisting of a non-covalent complex of three trimer submits (disulfide-bonded chains a, b and c) and three monomer subunits (chain d). X-ray diffraction of crystals of the dodecamer grown at neutral pH, were found to be monoclinic, with the unit cell dimensions: a = 112.3 A, b = 190.0 A, c = 69.6 A, beta = 102.0 degrees with h + k + l = 2n + 1 absent, characteristic of space group I121. In addition, these crystals exhibit a pseudo trigonal P321 symmetry with unit cell dimensions a = 190.5 A, b = 190.5 A, c = 69.5 A, gamma = 120.0 degrees. Assuming that the assymetric unit contains an entire dodecamer, a model of the latter was constructed that satisfies the symmetry of the trigonal pseudo cell and is consistent with the symmetry of the I121 crystallographic cell. The resulting model has strong implications concerning the hexagonal bilayer structure of the native hemoglobin.