THE leucine-rich repeat is a recently characterized structural motif(1) used in molecular recognition processes as diverse as signal transduction, cell adhesion, cell development, DNA repair and RNA processing(2). We present here the crystal structure at 2.5 Angstrom 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 beta-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.