The monoclonal antibody ER-MP58 recognizes an antigen present on all mouse macrophage colonystimulating factor (M-CSF) responsive cells in the bone marrow, including the earliest colony-forming myeloid progenitors, as well as by the majority of other myeloid precursors. The antigen is detected on a broad development range of macrophage precursor cells to the monocytic level, but also on granulocytes. Expression is rapidly lost upon maturation beyond the monocytic stage. The antigen disappears in the course of macrophage differentiation. Furthermore the antigen is clearly different from commonly used myeloid markers as Mac-1, F4/80, and Gr-1.The monoclonal antibody ER-MP58 is very useful for the identification of mouse myeloid hematopoietic islands in various organs, and for embryonic studies. Cells committed to the myeloid lineage can be separated from progenitor cells with other differentiation capacities by means of multiparameter cell sorting using monoclonal antibody ER-MP58 in combination with monoclonal antibody ER-MP12 (HM1084) and ER-MP-20 (HM1082).