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Six monoclonal antibodies (McAb) of the FMC series with specificity for granulocytic or monocytic cells were investigated by means of the immunogold method in combination with the myeloperoxidase reaction at ultrastructural level. This method allowed a precise characterization of the level of myeloid maturation at which the antigens detected by the McAb were expressed and confirmed their specificity for the two myeloid lineages. In addition, it facilitated the identification of granulocytic and monocytic cell components in cases of acute myeloid and chronic myelomonocytic leukemia. Blast cells did not react with any of the McAb used, and the reactivity in more mature cells was parallel to the development of cytoplasmic granules. The pattern of reactivity was similar in normal and leukemic cells, thus suggesting that these McAb could be applied to studies of normal myelopoiesis and to the classification of leukemia.

We have studied, by ultrastructural morphology and immunocytochemistry, the alterations that occur in cells from the HL60 leukaemia cell line and from patients with CGL following incubation in vitro with 1,25(OH) 2 D 3 for 2–5 days. The main morphological changes observed were in the nuclear shape, the development of autophagic vacuoles and the appearance of a population of small granules in the cytoplasm. These changes were associated with a significant reduction in MPO activity and increased expression of membrane antigens detected by the monocyte-specific McAb FMC17 and FMC32, as shown by the IGM at EM level, and a decrease in granulocyte-specific antigens demonstrated by the McAb FMC10. These observations suggest that promyelocytes and myelocytes could transform into monocyte-like cells and that this remodelling of cells was associated with autophagic digestion of cellular structures.