Described here is a new separation concept which theory shows to have considerable potential for analytical-scale work. Like chromatography, separation is achieved by differential migration in a narrow tube through which fluid is flowing unidirectionally. Unlike chromatography, separation is brought about in a one phase (or at least a continuous phase) system. It should not only be applicable to chromatography-type separations, but, more important, it should exhibit a distinct advantage in separating macromolecules and colloids, because of its essentially one-phase nature.