apo-Myoglobin (apo-Mb) was reconstituted with three copper complexes: Cu-II(Sal-Phe) (1; Sal-Phe = N-salicylidene-L-phenylalanato), Cu-II(Sal-Leu) (2; Sal-Leu = N-salicylidene-L-leucinato), and Cu-II(Sal-Ala) (3; Sal-Ala = N-salicylidene-L-alanato). The crystal structures of 1(.)apo-Mb (1.65 angstrom resolution) and 2(.)apo-Mb (1.8 angstrom resolution) show that the coordination geometry around the Cu-II atom in apo-Mb is distorted square-planar with tridentate Sal-X and a N-epsilon atom of His64 in the apo-Mb cavity and the plane of these copper complexes is perpendicular to that of heme. These results suggest that the apo-Mb cavity can hold metal complexes with various coordination geometries.