Abstract: An N-glycosylated glycoprotein, having the molecular weight of 105 kDa when glycosylated and 85 kDa when non-glycosylated, is present on the surface of BL3 hematopoietic stem cells, but is absent from the surfaces of other cells including 32D and FDC-P1 myeloid progenitor cells, EL4 T-cells and 3T3 fibroblasts. Antibody which binds the gp105 protein inhibits the proliferation of BL3 cells that occurs in BL3-conditioned medium (BL3CM). It also can inhibit preferentially the formation of fetal liver cell-derived, multi-lineage colonies in vitro, and the development of bone marrow cell-derived, spleen colony-forming unit foci in vivo. Anti-gp105 antibody can be employed in making a preparation that is enriched for hematopoietic stem cells. Substantially purified from other proteins, the gp105 protein itself is a useful reagent for isolating and characterizing the factor(s) responsible for the proliferative effect of BL3CM.