A device has been developed that is able to fractionate a mixture of microscopic particles according to small differences in their volume. A dilute suspension of the particles passes through a sensor within which the volume of each particle is electronically measured. Subsequently the suspension emerges into the air as a liquid jet. This jet is caused to break into a large number of individual droplets, thereby isolating the suspended particles. Droplets containing particles of the desired volume ranges are electronically charged and electrostatically deflected into suitable receptacles. The apparatus is described in considerable detail with illustrative application to polystyrene spheres and to human white blood cells.