Abstract: The invention provides a method of quantitating reticulocytes with high sensitivity and high accuracy at low cost employing a fluorescent dye suitable for the staining of reticulocytes. A blood sample stained with a fluorescent dye composition containing TOTO-3, TO-PRO-3, POPO-3, PO-PRO-3, YOYO-1, YO-PRO-1, YOYO-3 or YO-PRO-3 as the fluorescent dye is exposed to light in the red-wave length region in the case of TOTO-3 or TO-PRO-3, light in the green region for POPO-3 or PO-PRO-3, light in the blue region for YOYO-1 or YO-PRO-1, or light in the orange region for YOYO-3 or YO-PRO-3 and the resultant fluorescent emission from the blood sample is measured to enumerate reticulocytes.