1. A method for classifying and counting leukocytes, which comprises: (1) a step of staining cells in a sample obtained from a hematological sample by treatment with a hemolytic agent, with a fluorescent dye which can make a difference in the fluorescence intensity at least among mature leukocytes, leukocytes with abnormal DNA amount and immature leukocytes; (2) a step of introducing the sample containing the stained cells into a flow cytometer to measure scattered light and fluorescence of the respective cells; (3) a step of classifying a first group and a second group utilizing a difference in the intensity of a scattered light peak and a difference in the scattered light width, the first group including leukocvtes and second group including coincidence cells and platelet clumps; (4) a step of classifying and counting mature leukocytes, leukocytes with abnormal DNA amount and immature leukocytes, utilizing a difference in the scattered light intensity and a difference in the fluorescence intensity of leukocytes classified in the step (3).