A method and a device employing the method of flow cytometry, preferably applicable but not limited to the counting and differentiation of leukocytes. It relates more particularly to the field of simplified haematology instruments with moderate operating costs. The method is characterized in that a technique of impedance measurement is used for identifying the particles whose trajectory did not pass through a predetermined optical measurement zone in order to process them selectively, thus avoiding the use of sheath fluids for guiding the particles towards the measurement zone.