A reagent for classifying leucocytes including: a) at least one of surfactant selected from a cationic surfactant, an amphoteric surfactant, and the combination thereof; b) an organic compound having phenyl or heterocyclic group, wherein when the surfactant is a cationic surfactant, the organic compound is an organic compound having an anionic group; when the surfactant is an amphoteric surfactant or a combination of amphotheric and cationic surfactant, the organic compound is an organic compound having an anionic or cationic group; c) a buffer for adjusting pH.