Studies on the early events in the differentiation of the nonspecific immune system require the identification and isolation of myeloid‐committed progenitor cells. Using the monoclonal antibodies (mAb) ER‐MP12 and ER‐MP20, generated against immortalized macrophage precursors, we have shown previously that the earliest macrophage colony‐stimulating factor (M‐CSF)‐responsive cells in the bone marrow have the ER‐MP12^hi20⁻ phenotype. In addition, we found that the ER‐MP12^hi20⁻ subset (comprising about 2 % of total nucleated marrow) contains progenitor cells of all hematopoietic lineages. Aiming at the identification and purification of the myeloid progenitor cells within the ER‐MP12^hi20⁻ subset, we used ER‐MP58, a marker expressed at high level by all M‐CSF‐responsive bone marrow progenitors. With this marker the ER‐MP12^hi20⁻ cell population could be divided into three subfractions: one with absent or low level ER‐MP58 expression, one with intermediate, and one with high ER‐MP58 expression. These subfractions were isolated by fluorescence‐activated cell sorting and tested in vitro and in vivo for their differentiation capacities. In addition, the expression of ER‐MP58 on stem cell subsets was examined in the cobblestone area‐forming cell (CAFC) assay. Our data indicate that in the ER‐MP12^hi20⁻ subpopulation myeloid‐committed progenitors are characterized by high‐level expression of the ER‐MP58 antigen, whereas cells with other or broader differentiation capacities have an ER‐MP58 negative/low or intermediate phenotype. These myeloid‐committed progenitors have no significant repopulating ability in vivo, in contrast to the ER‐MP58 intermediate cells. Primitive CAFC‐28/35, corresponding to cells providing long‐term hematopoietic engraftment in vivo, also did not express the ER‐MP58 Ag at a high level. Thus, cells committed to the myeloid lineage can be separated from progenitor cells with other differentiation capacities by means of multiparameter cell sorting using ER‐MP58 in combination with ER‐MP12 and ER‐MP20.