Two-dimensional crystals of soluble proteins were formed at the interface between an aqueous solution of proteins and a thin organic liquid (dehydroabietylamine). Proteins were adsorbed to the interface from the aqueous side and formed a two-dimensional crystal under suitable conditions. This method offers the advantage of great surface mobility and ideal homogeneity. Furthermore, the positive charge attracted negatively charged proteins well to the interface and no denaturation of the proteins was observed. With this technique, two-dimensional crystals of ferritin, catalase, chaperonin and 50S ribosome were prepared and their structural features were determined.