We have used three-color flow cytometry to investigate the pattern of expression of the CD11/CD18, CD44, and leukocyte adhesion molecule 1 (LAM-1) adhesion molecules during myeloid and erythroid differentiation in humans. The earliest myeloid cells, identified as CD33loCD15-, were exclusively CD44hi but contained both leukocyte function-associated antigen 1 (LFA-1hi) and LFA-1lo cells, as well as LAM-1+ and LAM-1- cells. This CD33loCD15- myeloid subpopulation expressed only low levels of CD11c and failed to express CD11b, CD14, or any lymphoid (CD3, CD16, CD19) antigens or glycophorin. Commitment to monocyte differentiation, suggested by the presence of an LFA-1hi CD11c+ subset within the CD33loCD15- subpopulation, was clearly signaled by upregulation of CD33; these monocyte-lineage committed cells were exclusively CD33hi, CD44hi, CD11ahi, CD11c+, and exhibited a broad range of intensity of CD15 expression. Later stages of monopoiesis were identified by acquisition of CD11b, and subsequently of CD14. Myeloid cells committed to granulopoiesis remained LFA-1lo, and underwent a sharp upregulation of CD15 along with downregulation of both CD33 and CD44. Successive stages of granulocyte development were marked by expression of CD11b and, subsequently, of CD16. The earliest cells capable of erythroid differentiation were CD44hi, LFA-1lo, and LAM-1+. Both LFA-1 and LAM-1 were lost before the onset of glycophorin (glyco) expression, whereas CD44 expression remained high on glyco+ cells, which also expressed CD45. CD44 expression was intermediate on glyco+ CD71+ cells, and low on glyco+ CD45- CD71- cells, similar to normal, circulating erythrocytes. Our results allow us to phenotypically define discrete stages in the normal development of monocytes, neutrophils, and erythrocytes. The expression of LFA-1, LAM-1, and high levels of CD44 on the most primitive hematopoietic cells detectable by flow cytometry suggests that at least some of these molecules are critically involved in leukocyte adhesion during development.