1. A method for classifying and counting leukocytes by an optical means which comprises; (i) treating a blood sample containing leukocytes with an aqueous solution having a pH ranging from 5.0 to 11.0 and comprising an ionic surfactant and a fluorescent dye, to give an optical difference between eosinophils and other leukocytes, wherein the ionic surfactant is used at a concentration effective to not damage the whole of the cell membrane of leukocytes in the blood sample but effective to damage a part of the cell membrane so as to allow the fluorescent dye to enter the leukocytes and stain intracellular material, and wherein said ionic surfactant is at least one selected from the group consisting of: a) a quaternary ammonium salt of the following formula: ##STR7## wherein R.sub.1, R.sub.2 and R.sub.3 are independently a hydrogen atom, a C.sub.1 -C.sub.8 alkyl group or a C.sub.6 -C.sub.8 aralkyl group; R.sub.4 is a C.sub.8 -C.sub.18 alkyl group, a C.sub.8 -C.sub.18 alkenyl group or a C.sub.8 -C.sub.18 aralkyl group; and X.sup.- is an anion, b) a pyridinium salt surfactant of the following formula: ##STR8## wherein R5 is a C8-C18 alkyl group; and X.sup.- is an anion, and c) an amphoteric surfactant of the following formula: ##STR9## wherein R.sub.6 and R.sub.7 are independently a hydrogen atom or a C.sub.1 -C.sub.8 alkyl group; R.sub.8 is a C.sub.8 -C.sub.18 alkyl group; and n is 1 or 2; and (II) classifying and counting the leukocytes by measuring an optical difference between eosinophils and other types of leukocytes in the blood sample based on at least two kinds of light selected from the group consisting of forward scattered light, side scattered light, red fluorescence light and green fluorescence light.