Abstract: A leukocyte classification reagent comprising at least one surface active agent selected from the group consisting of anionic active agents and amphoteric active agents lyses erythrocytes within a short period of time when added to a blood sample but does not cause damage on leukocytes for a certain period of time keeping their original state or close to the original state so that leukocytes can be classified directly into four groups, namely lymphocyte, monocyte, neutrophil and eosinophil, by measuring forward scattering and side-way scattering by a flow cytometer after lysis of erythrocytes.