A possibility of crystallography to estimate large conformational change of proteins is described. An example is the crystal structure of a mutant T4 lysozyme (Faber & Matthews, 1990), where four independent molecuels show a variety of hinge-bending angles. Another example is human α-lactalbumin (Harata & Muraki, 1992) . The amino acid residues of 104-110 assume α-helix in one crystal and a loop structure in another crystal. These observations demonstrate the large conformational flexibility of proteins and indicate that the X-ray crystallography is a powerful tool for the study of dynamic structure of proteins.