A water-soluble protein (ferritin) was spread in 0.15 M sodium chloride and mixed with a surfactant solution of tetraethylene glycol octyl ether or pentaethylene glycol decyl ether. The mixed solution was then spread on a subphase (pH 4.1) of a 0.1 M acetate buffer with sodium chloride, which does not dissolve ferritin but does dissolve the surfactants. Ferritin, with aid of the surfactants as a spreading agent, formed a monolayer in two dimensions with an amorphous ordering. The inclusion of 5% cadmium sulfate in the subphase permitted the formation of ferritin crystals in three dimensions on the subphase surface. A decrease in the concentration of cadmium sulfate in the subphase solution yielded a two-dimensional ordered array of ferritin molecules.