We developed a new microfluorometric method to measure the DNA and RNA contents of individual megakaryocytes using acridine orange (AO) staining in human bone marrow smears. Some bone marrow smears were fixed with ethanol and treated with pretreatment solution and then stained with an AO staining solution. The DNA content was assayed by measuring the green fluorescence and the RNA content was assayed by measuring the red fluorescence under B excitation with microfluorometer using peripheral lymphocytes as the control. The ploidy peak was shown to be 16N in all of 5 normal controls, but it was 8N in the patient with refractory anemia with excess of blasts in transformation (RAEB-T). There was not difference in the RNA content/DNA content ratios (RI) between each ploidy in the normal controls. The RI of the patient with RAEB-T was lower than that of the normal controls. The measurement of the DNA-RNA contents may be useful as a new megakaryocytic parameter.