The megakaryocytic ploidy was microfluorometrically measured in 12 normal controls and 15 myelodysplastic syndrome (MDS) patients using DAPI (4′,6-diamidino-2-phenylindole) staining after destaining of the Wright-Giemsa (WG) stain. MDS patients had slightly more immature megakaryocytes when compared with normal controls. The megakaryocytic ploidy distribution had a peak at 16N in normal controls, at 8N in the 11 of the 15 MDS patients, and at 4N in the remaining 4 patients, which is suggestive of impaired polyploidization in MDS patients. In MDS, micromegakaryocytes were shown not to be immature but much more impaired in polyploidization than non-micromegakaryocytes. However, there was no difference in the megakaryocytic ploidy pattern among the type of the modification of Feinendegen' classification in each case for both the normal controls and the MDS patients, suggesting that the megakaryocytic ploidy is probably determined at the maturation level of the megakaryoblasts or the precursor cells. The study of megakaryocytic ploidy before and after therapy in the case of refractory anemia with excess of blasts might suggest that the remission of MDS patients might be qualitatively different from that seen in acute leukemia patients. Furthermore, the DNA histogram of the megakaryocytes from one of the two MDS patients obtained by the new method, which is able to determine the amount of DNA in the immunologically identified megakaryocytes microfluorometrically, using the monoclonal anti-glycoproteins IIb/IIIa antibody on bone marrow smears, showed a shift towards small ploidy compared with those defined on the basis of WG staining. This finding indicates that the micromegakaryocytes or the megakaryoblasts which could not be identified morphologically can be identified immunologically. This method may be useful, especially in the cases with pathological megakaryocytes and megakaryocytic precursor cells such as in MDS.