AbstractThe molecular structure of interleukin‐1β, a hormone‐like cytokine with roles in several disease processes, has been determined at 2.0 Å resolution and refined to a crystallographic R‐factor of 0.19. The frame‐work of this molecule consists of 12 antiparallel β‐strands exhibiting pseudo‐3‐fold symmetry. Six of the strands make up a β‐barrel with polar residues concentrated at either end. Analysis of the three‐dimensional structure, together with results from site‐directed mutagenesis and biochemical and immunological studies, suggest that the core of the β‐barrel plays an important functional role. A large patch of charged residues on one end of the barrel is proposed as the binding surface with which IL‐1 interacts with its receptor.