The characterization of early branch points in the differentiation of leukocytes requires identification of precursor cells in the bone marrow. Recently, we produced two monoclonal antibodies, ER‐MP12 and ER‐MP20, which in two‐color flow‐cytometric analysis divide the murine bone marrow into six defined subsets. Here we show, using fluorescence‐activated cell sorting followed by macrophage colony‐stimulating factor‐stimulated culture in soft agar, that precursors of the mononuclear phagocyte system reside only within the ER‐MP12^hi20⁻, ER‐MP12⁺20⁺ and ER‐MP12⁻20^hi bone marrow subsets. Together, these subsets comprise 15% of nucleated bone marrow cells. Furthermore, we provide evidence that the macrophage precursors present in these subsets represent successive stages in a maturation sequence where the most immature ER‐MP12^hi20⁻ cells develop via the ER‐MP12⁺20⁺ stage into ER‐MP12⁻20^hi monocytes.