A reagent for classification of leukocytes includes (a) at least two cationic surfactants; (b) at least one organic compound bearing a hydrophobic group and an anionic group; (c) a buffer for adjusting pH into a range of approximately 2-8. Also disclosed is a method for classifying leukocytes with the reagent. With the reagent and method, erythrocytes are lysed rapidly and classification of leukocytes into five groups is achieved in the same channel. The reaction may be carried out at approximately between 10-40° C. and scattered light signals may be detected at two angles for measuring the classification of leukocytes into five groups.