The characteristics of nine monoclonal antibodies (MoAbs) produced using the uninduced cells of a human erythroleukemia line (HEL) as immunogen are described. These antibodies were grouped into four categories by their differences in recognition of normal cells and cells of hemopoietic cell lines. The four MoAbs of group A recognize determinants that are expressed in a large proportion of normal bone marrow cells and other mature cells. The two MoAbs of group B (53/5, 53/6) and the two MoAbs of group C (54/23, 54/39) recognize small proportions of bone marrow cells, whereas the single MoAb of group D (53/10) essentially recognizes only HEL cells. Competition experiments revealed two pairs of competing Abs (53/5 and 53/6; 54/23 and 54/39). In complement-dependent cytotoxicity of progenitors, 53/6 produced 90%- 100% inhibition of CFU-E, BFU-E, and CFU-C growth; 54/39 30%–60% inhibition of BFU-E and CFU-C growth; 53/10 produced a variable degree of inhibition of CFU-E and BFU-E. Cell sorting using 53/6 resulted in approximately a 10–12-fold enrichment of CFU-E, BFU-E, and CFU-C among the positive cells. Cell sorting with 54/23 resulted in recovery of over 90% of BFU-E and 100% of CFU-C among the 23.5% of sorted cells showing strong or intermediate positivity. These findings suggest that HEL cells possess surface characteristics that are expressed in several classes of hemopoietic progenitors.