The present invention demonstrates that M-CSF responsiveness and the M-CSFR expression can be used to discriminate monocytic and granulocytic cells within a population of cells which strongly expresses the CD34 antigen (CD34.sup.hi). Briefly, the method comprises isolating phenotypically and functionally defined CD34.sup.+ subsets, and staining with anti-M-CSFR monoclonal antibodies to measure expression on these primitive progenitors and cells committed to the granulocytic and monocytic lineages, based upon expression of M-CSFR. CD34.sup.hi M-CSFR.sup.hi cells are highly clonogenic and approximately 70% of the colonies are CFU-M (monocytic), whereas less than 20% were CFU-G (granulocytic). In contrast, CD34.sup.hi cells that were positive for the granulo-monocytic marker CD64 and negative for the M-CSFR contained high frequencies of 91% pure CFU-Gs. After 60 h in culture, CD34.sup.hi M-CSFR.sup.hi cells developed into distinct populations of M-CSFR.sup.hi and M-CSFR.sup.lo cells. These two populations gave rise almost exclusively to monocytes and granulocytes, respectively. This result demonstrates that M-CSF target specificity among human hematopoietic progenitor cells is determined by lineage-specific regulation of the M-CSFR and demonstrate that M-CSFR is a useful marker to discriminate CFU-Ms from CFU-Gs.