The conformational structure of four distinct crystals of b-lactoglobulin, lattices K, X, Y and Z, have been studied by x-ray crystallography and the structures of lattices X, Y, and Z, have been determined. Each of these forms has so far given rise to electron density maps at a resolution of 2.8 A or better. Interpretation of these maps has shown that the structure consists of nine strands of anti-parallel b-sheet and one short a-helix. The core of the molecule is made up of a very short a-helix segment and eight strands of anti-parallel b-sheet, which wrap round to form an antiparallel b-barrel.