Immunological analysis of bone marrow cells in myelodysplasia using immunofluorescence did not allow accurate morphological identification of blast cells. However, improvement of the immunoperoxidase technique allows one to realize the diagnostic potential of immunocytochemistry. CD34 immunotyping of blasts in normal human bone marrow showed 0.8 +/- 0.4% CD34 positive blasts and these cells had the morphology of type 1 blasts. The increase of bone marrow blasts in RAEB patients is related to CD34 negative type II and III blasts. A clone of undifferentiated CD34 positive blasts is characteristic of RAEB-T and acute myeloid leukaemia evolving from myelodysplasia. The detection of CD34 positive bone marrow blasts allows a better discrimination between RAEB and RAEB-T.