The thesis deals with different erythrocyte lysis reagents (with / without fixative) and whith their influence on leukocytes, as well as with the use of flow cytometry for the differentiation of various leukocyte subpopulations. Erythrocytes in samples interfere with the performance of the analysis due to the average concentration of leukocytes on an average of 1000 times and must therefore be eliminated. This is done by adding different lysis reagents. These solutions affect not only the red but also the white blood cells. Cell losses of individual leukocyte subpopulations, such as CD34-positive stem cells / blasts, have been described by many authors (Einwallner E. et al., 2013; Menéndez P. et al., 1998; Alvarez-Larran A. et al., 2002; Gratama JW et al., 2000). Losses are due on the one hand to the nature and composition of each lysis reagent, which has an aggressive effect on some leukocytes, but also on the strength of centrifugation and other methodological differences in the workup, such as individual washing steps. Cell loss could cause misdiagnosis, which is why newly established reprocessing steps must be precisely evaluated before they are used. Bone marrow (BM), as a sensitive sample material, which is available in smaller quantities, requires a particularly careful preparation, and in the case of implausible results, an limited number of repeat measurements can be carried out. Comparative measurements of individual important leukocyte subpopulations are an established method to compare different treatment methods. In particular with regard to bone marrow, the comparison should also include rarer cell populations that can only be found in this material and that are of high importance for many diagnoses. Such a comparison of BM sample preparation by applying two different methods will be part of the bachelor thesis 2.