We have analyzed the structure of the interface between VL and VH domains in three immunoglobulin fragments: Fab KOL, Fab NEW and Fab MCPC 603. About 1800 A2 of protein surface is buried between the domains. Approximately three quarters of this interface is formed by the packing of the VL and VH beta-sheets in the conserved "framework" and one quarter from contacts between the hypervariable regions. The beta-sheets that form the interface have edge strands that are strongly twisted (coiled) by beta-bulges. As a result, the edge strands fold back over their own beta-sheet at two diagonally opposite corners. When the VL and VH domains pack together, residues from these edge strands form the central part of the interface and give what we call a three-layer packing; i.e. there is a third layer composed of side-chains inserted between the two backbone side-chain layers that are usually in contact. This three-layer packing is different from previously described beta-sheet packings. The 12 residues that form the central part of the three observed VL-VH packings are absolutely or very strongly conserved in all immunoglobulin sequences. This strongly suggests that the structure described here is a general model for the association of VL and VH domains and that the three-layer packing plays a central role in forming the antibody combining site.