Disclosed is a reagent for use in the analysis of a primitive leukocyte, which comprises a surfactant capable of damaging a cell membrane of an erythrocyte or a mature leukocyte, a solubilizer capable of shrinking a damaged blood cell and a nuclei-acid-staining dye, and which has an osmotic pressure of not lower than 10 mOsm/kg and lower than 150 mOsm/kg.