THE leucine-rich repeat is a recently characterized structural motif¹ used in molecular recognition processes as diverse as signal transduction, cell adhesion, cell development, DNA repair and RNA processing². We present here the crystal structure at 2.5 Å resolution of the complex between ribonuclease A and ribonculease inhibitor, a protein built entirely of leucine-rich repeats. The unusual non-globular structure of ribonuclease inhibitor, its solvent-exposed parallel β-sheet and the conformational flexibility of the structure are used in the interaction; they appear to be the principal reasons for the effectiveness of leucine-rich repeats as protein-binding motifs. The structure can serve as a model for the interactions of other proteins containing leucine-rich repeats with their ligands.