Only mature leukocytes can be found in the healthy people’s blood. In some diseases both mature cells and younger cells may pass into the peripheral blood. Leukocytes counts in blood take between 3.4 x 109/L and 9.7 x 109/L. Values under 3 x 109/L present leukopenia, and over 10 x 109 /L leukocytosis. Leukocytes can be determined in the sample of venous or capillary blood. It is recommended to determine leukocytes count in the venous blood. The leukocytes count can be measured by a hemocytometer or by an electronic cell counter. Leukocytes counts are performed on blood samples appropriately diluted with the Türck solution that lyses the erythrocytes and stained with gentian-violet dye that stains nuclei of all cells in the blood sample. The principle of determination leukocytes counted by the hematology counters is the same as for erythrocytes.