In our study we used a new proposed system of CD45 monoclonal antibody in combination with the side scatter (SSC) parameter as a very useful gating method allowing myeloblast detection especially in cases with low blasts percentage in examined samples. Immunological demonstration of myeloperoxidase (MPO) in the cytoplasm of AML blasts is considered to be a reliable and highly sensitive marker. Using a direct single and double immunofluorescence staining method and flow cytometry we evaluated the intracellular expression of two granular constituents of myeloid cells--MPO and lactoferrin (LF) in leukemia cells from 18 patients at AML diagnosis, two patients in remission after allogenic bone marrow transplantation and in six controls. Two different fixation/permeabilization techniques were used: Fix&Perm, paraformaldehyde and saponin prior to monoclonal antibody staining in order to verify the sensitivity of two labeling methods for MPO. Although both reagents used in this study proved to be efficient tools for the fixation and permeabilization of leukemia cells, the second one was characterized by higher sensitivity in detection of MPO. By double staining of MPO and LF we were able to distinguish undifferentiated cells from the granulomonocytic maturation compartments in bone marrow, since LF is proposed to be selectively expressed from the myelocyte stage of differentiation onward. Cytoplasmic CD13 expression was detectable in AML blasts after their buffered-formaldehyde-acetone fixation/permeabilization. According to our results the detection of MPO and CD13 markers in the cytoplasm of leukemia cells is of great importance in the definition of FAB M0-M1 subtype of AML. Furthermore we described overexpression of CD34 antigen in AML and revealed the characteristic marker combination when CD34 was studied simultaneously with MPO. This finding also coincided with some atypical phenotypic features (CD15/MPO, CD7/cCD13, CD2/cCD13, CD33/cCD13, MPO/cCD13) contributing to the differential diagnosis and allowing the immunologic monitoring of patients for the presence of residual disease.