A rat monoclonal antibody, YBMi42, directed against mouse leukocyte common antigen, was used for the analysis and separation of hemopoietic progenitor cells from mouse bone marrow and fetal liver. Cells were fractionated o n a FACS-II cell sorter and the resulting subpopulations examined for their morphology and ability to form colonies in agar (for day 7 colonies) and methylcellulose (for day 2 erythroid clones). The antibody bound to all leukocytes, including blast cells and day 7 hemopoietic progenitor cells (day 7 colony forming cells, CFC), but not to erythrocytes or nucleated erythroid cells. This antibody can be used to advantage to enrich for early progenitor cells from mouse fetal liver, in which the majorityof cells (70%) are nucleated erythroid cells. In day 12 fetal liver, approximately 10% of all cells bind this antibody strongly and, of these approximately 70% are blast cells. Contained within this positive population are 95% of all day 7 CFC. In the most enriched fraction about 20% of t h e cells formed day 7 colonies. This represents a 25fold enrichment over unsorted fetal liver. The negative fractions contain 94% of all cells forming erythroid clones ( 2 8 cells) o n day 2 of culture (day 2 CFU-E). In the most enriched fraction, 20% of the cells are day 2 CFU-E. Day 7 CFC can therefore be well separated from day 2 CFU-E, with good recovery of both cell types, by use of a single label. Day 7 colony forming cells were classified as granulocyte (G-CFC), macrophage (M-CFC), mixed granulocyteimacrophage (GM-CFC), pure erythroid (El, or mixed erythroid (E,J. A high enrichment for multipotential cells is achieved and constitutes 3-5% of cells in the most enriched fraction. Most types of day 7 CFC could not be separated with YMBi42, but GM-CFC and M-CFC exhibit a broader distribution than t h e other CFC with regard to fluorescence intensity. This implicit heterogeneity in GM-CFC and M-CFC is further substantiated by the finding that myeloid progenitors in the different FACS fractions also share a differential reactivity to different sources of growth factors.