The aim of this study was to identify markers specific for granulo-monocytic commitment of progenitor cells. Large panels of antibodies were screened for selective staining of subsets of CD34+ cells from fetal and adult bone marrow. Flow cytometric analysis showed that CD64/fc gamma RI was undetectable on noncommitted progenitor cells (CD34++, CD38-/lo, HLA-DR+) and expressed on a subset of lineage-committed progenitors (CD34+, CD38+) with higher mean orthogonal light scatter than the remaining CD34+ cells. The CD34+, CD64+ cells were CD19- and the majority were CD45RA+, CD71lo, suggesting that CD64 recognized granulomonocytic progenitor cells. Specificity of CD64 for the granulo-monocytic lineage was shown by demonstrating that colonies arising from CD34+, CD64+ cells consisted of 98% +/- 2% colony-forming unit-granulocyte-macrophage (CFU-GM) in semisolid medium containing stem cell factor (SCF), interleukin-3 (IL-3), IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF), and erythropoietin (EPO). In contrast, 63% +/- 15% of the colonies from the CD34+, CD64- cells were burst-forming unit-erythroid/colony-forming unit-erythroid (BFU-E/CFU-E). Furthermore, four-color immunofluourescence and cell sorting was used to analyze the progeny of cells cultured in liquid medium containing identical cytokines as used in the semisolid medium. This analysis showed that CD34+, CD64+ cells gave rise to 83% +/- 10% granulo-monocytic cells whereas progeny of the CD34+, CD64- cells contained 81% +/- 11% erythroid cells. Neutrophils as well as basophils and monocytes/macrophages were present in the cultures from CD34+, CD64+ cells, showing that this population contains progenitors of most types of granulo-monocytic cells. Two widely used myeloid markers, CD13 and CD33, were not myeloid-specific, because both were clearly positive on noncommitted progenitor cells. Of 40 antigens tested, CD15 was the only other marker fulfilling the criteria of a myeloid-specific marker. However, at concentrations of CD15 that did not induce aggregation, CD15+ cells constituted less than 50% of the CD34+, CD64+ cells. Furthermore, the CD34+, CD15- cells showed more than 50% higher CD34 mean fluorescence intensity than the CD64+, CD15+ cells, indicating that CD64 appears earlier than CD15 during differentiation. Thus, among a large number of antigens screened, CD64 was the most useful for the identification and purification of granulo-monocytic progenitor cells.