Recombinant human interleukin-5 (rhIL-5) has been crystallized by the hanging drop vapor diffusion method using 0.1 M-Tris.HCl buffer (pH 8.5) containing 0.2 to 0.25 M-sodium acetate and 26 to 30% PEG 4000 at 22 degrees C. The parallel-piped crystals belong to the space group C2 with unit cell dimensions of a = 122.1 A, b = 36.11 A, c = 56.42 A, beta = 98.59 degrees. They diffract to at least 2.0 A resolution on a rotating anode X-ray source. The molecular mass weight of the protein and the volume of the unit cell suggest that the asymmetric unit contains one intermolecular disulfide-bonded homodimer.