is blended with the blood in such an amount followed by the centrifugation of the resulting mixture and the subsequent removal of the resulting supernatant these operations are repeated one to several times then the washing analysis liquid is again added to the resulting residue in an amount ranging from 0 5 to 10 times preferably 1 to 4 times that of the blood to be treated followed by mixing them and the analysis thereof according to the flow cytometry technique in this respect when it takes not less than 24 hours till the measurement is initiated it is desirable to use a washing analysis liquid which comprises an antiseptic and or an agent for inhibiting the flow ability of the cytoplasmic membrane such as sodium azide or cytochalasin b the blood used in the kit according to the present invention may be one immediately after the collection thereof one stored in for instance an ice or a refrigerator within a short period of time or one stored in a frozen condition at a temperature of not higher than u2212 30 u00b0 c in this respect the blood sample stored in a frozen condition is used after thawing the same by the usual method upon the practical use thereof the kit of the present invention can be applied to the staining of not only the mononuclear cells present in the blood but also other cell suspensions more specifically the kit of the present.