The oxidized form of purple acid phosphatase from pig allantoic fluid has been crystallized in the presence of phosphate using the hanging-drop technique. The crystals belong to the space group P212121 and have unit-cell parameters a = 66.8, b = 70.3, c = 78.7 Å. Diffraction data collected from a cryocooled crystal using a conventional X-ray source extend to 1.55 Å resolution. A knowledge of the three-dimensional structure of mammalian purple acid phosphatase will aid in understanding the substrate specificity of the enzyme and will be important in the rational design of inhibitors, with potential in the treatment of bone diseases.