Crystals of human erythrocytic purine nucleoside phosphorylase have been grown from solutions of ammonium sulfate. The crystals are trigonal, space group R32; the hexagonal axes are a = 143.8(2) and c = 165.1(2) A. The crystals are moderately stable to x-rays and diffract beyond 3.0 A resolution. The experimental density of the crystals indicates that the molecular weight of the protein is 94,000. The three subunits are not related by crystallographic symmetry.