Abstract: A novel composition of matter, process and apparatus is provided for distinguishing and absolutely counting leukocytes, erythrocytes, reticulocytes, and other cells as well as differently counting and classifying the different types of leukocytes. The composition of matter comprises a plurality of compatible dyes for uniquely distinguishing leukocytes, erythrocytes, reticulocytes, and other cells randomly distributed in a medium. At least one of the dyes imparts a characteristic fluorescence to all leukocytes stained therewith. The nature of the dye composition is such that when the stained leukocytes are irradiated with radiation having the characteristic absorption wavelengths of each dye, the relative intensities of the light emitted and/or transmitted by each leukocyte type defines that particular leukocyte type.