A reagent for classifying and counting leukocytes containing (1) a cyanine fluorescent dye; and (2) a glycoside compound; a reagent kit containing the reagent for classifying and counting leukocytes as well as its preparation process; and a process for classifying and counting blood cells using the reagent or kit are provided. Using the reagent, kit and/or process provided, leukocytes can be classified and counted in four groups with a high degree of differentiation and a better classification among each subpopulation of leukocytes, especially in that it successfully addresses the indistinct classification between lymphocytes and monocytes and between the eosinophils and neutrophils in a scattergram.