Interleukin-8 (neutrophil-activating factor; NAP-1) has been crystallized by the vapour diffusion technique to give single crystals suitable for three-dimensional structural study at a resolution higher than 2.4 A. The crystals belong to the space group P3(1)21 or P3(2)21 and have unit cell dimensions a = b = 40.9 A, c = 90.3 A.