AbstractThe class I major histocompatibility (MHC) antigen HLA‐B27 was purified by immunoaffinity chromatography from the homozygous human B lymphoblastoid cell line LG‐2. Detergent‐soluble HLA‐B27 was cleaved with the protease papain to remove the hydrophobic transmembrane region and the cytoplasmic tail. Crystals of the resulting water‐soluble extracellular fragments were obtained in hanging drops by the vapor‐diffusion method. The crystals are triclinic, space groupP1, with unit cell dimensionsa= 45.9 Å,b= 71.0 Å,c= 83.7 Å, α = 79.4°, β = 88.5°, γ = 89.9°, and diffract bey