A method for classifying leukocytes in animal blood is described. In the method, a measurement sample is prepared by mixing a canine or feline blood sample with a lysing reagent. Erythrocytes are lysed and leukocytes are shrunk in the measurement sample. The data correlated with the size of leukocytes in the measurement sample are measured. The leukocytes, on the basis of the measured data, are classified into a first group containing lymphocytes, a second group containing neutrophils and monocytes and a third group containing eosinophils.