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.