Abstract: A hematological specimen for classifying and counting leukocytes with a flow cytometer is prepared. A sample to be assayed is prepared by eliminating influences of erythrocytes from a hematological sample without changing leukocytes morphologically by adding a first aqueous solution of a low osmotic pressure including a buffer for adjusting the pH value within an acidic region and a second aqueous solution including an osmolarity compensating agent and a buffer for giving pH value suitable for staining, optionally further adding a salt, which dissociates into ions in aqueous solutions so as to control the electrical conductivity of the aqueous solution at a preferable level, while damaging the cell membranes of erythroblasts contained in said sample; and staining the leukocytes with at least four dyes including Astrazon Yellow 3G and Neutral Red.