Monoclonal antibody ER-MP12 reacts with the major population of colony forming unit macrophage (CFU-M) precursor cells, with a subpopulation of pre CFU-M and monoblasts, and with precursor cells of granulocytes. The antibody can also be used, in combination with the monoclonal antibody ER-MP20, for identification of different stages of myeloid progenitor cell development.