Myeloperoxidase was purified from normal human leukocytes in a crystalline state. Two types of crystals were obtained by the batchwise and dialysis crystallization methods, one of which had a bipyramidal shape belonging to the orthorhombic system. Three multiple forms of human myeloperoxidase were separated from the crystalline enzyme by CM-Sepharose chromatography with sodium chloride gradient elution. These three multiple forms were found to have very similar enzymatic, spectroscopic, and chemical properties. However, slight differences were observed in their amino acid compositions and the molecular weights of their large subunits determined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Hemi-myeloperoxidase was prepared from holo-myeloperoxidase by reduction with dithiothreitol and modification with iodoacetamide, and the molecular shapes of the holo- and hemi-enzymes were determined by analytical ultracentrifugation. The axial ratios were calculated to be 2.4-3.5 for the holo-enzyme and 2.9-3.1 for the hemi-enzyme. These results suggest that the shapes of the two enzymes are more spherical in solution than the proposed structural model previously reported.