Abstract: The present invention provides a reagent for analysis of immature leukocytes comprising: a surfactant which can damage cell membranes of erythrocytes and mature leukocytes, a solubilizing agent which can shrink the damaged blood cells and a dye for staining nucleic acid, wherein the reagent has an osmotic pressure of not lower than 10 mOsm/kg and lower than 150 mOsm/kg.