In the leukocyte differential, morphological differential has been usually performed with chromatoaffinity of blood cells, but physical differential is recently performed with biophysicochemical characteristics of blood cells using an automated blood cell counter. The trend of the leukocyte differential has changed to physical differential (automated count method) from morphological differential (eye count method). In the automated count method, leukocytes are differentiated into neutrophils, lymphocytes, monocytes, eosinophils and basophils according to the predetermined region of each leukocyte population on the cytogram using an automated blood cell counter. An automated count method is very useful for the screening test in the laboratory because of its high precision and accuracy for the normal samples. However, it is not easy to identify abnormal cells such as leukemia cells by an automated count method. Therefore, an automated count method dealing with various suspect flags generated using an automated blood cell counter must be used for such abnormal samples. In case of absolutely few leukemia cells in the peripheral blood during complete remission, the automated count method can not detect a leukemia cell and no suspect WBC flags are recognized by the automated blood cell counter. Development of an automated blood cell counter with a higher efficiency than now and a standard automated count method is awaited in the clinical laboratory.