A large number of monoclonal antibodies (McAbs) directed against components on myeloid (granulocytic/monocytic) cells have been generated. Individual McAbs were identified which are selectively reactive with antigenic determinants expressed by myeloid cells at specific stages of differentiation in a lineage-restricted fashion. The composite phenotype obtained by a combination of antimyeloid McAbs allows for a precise definition of the normal or malignant cell type under investigation. Cell binding studies on normal and leukemic cells and the biochemical characterization of the antigens provided the basis for a grouping of those antimyeloid McAbs into clusters of differentiation (CD). The reactivity patterns of CD11, CD13, CD14, CD15, and CD33 McAbs and the characteristics of the respective antigens are reviewed. These CD McAbs distinguish leukemic cells of myeloid from those of lymphoid origin. The monocytic nature of AML cells can be recognized by CD14 McAbs, whereas the other CD McAbs react with both monocytic and nonmonocytic types of acute myeloid leukemia. The expression of these differentiation antigens is not concordant with the morphological-cytochemical French-American-British (FAB) classification of leukemia; nevertheless, tendencies for agreement are apparent. If used in combination, FAB typing and immunophenotyping could provide complementary information. Their potential use for mapping of myeloid differentiation and for cell type recognition in leukemia phenotyping demonstrates the utility of antimyeloid CD McAbs for biological or clinical investigations. The diagnostic value of antimyeloid McAbs is enhanced if the reagents are included in a panel of McAbs standardized for routine immunophenotyping.