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We enriched bone marrow cells from 10 normal individuals for primitive hematopoietic progenitors using a two-step technique, and examined resultant primitive progenitors for their in vitro long-term repopulating capacity and their ability to adhere to irradiated stroma. Immunomagnetic depletion of mature myeloid and lymphoid progenitors resulted in a lineage-negative (Lin-) cell population. Subsequent dual-color fluorescence activated sorting of cells with low forward and vertical light scatter properties, expressing CD34 antigen (34+) and either bearing (DR+) or lacking (DR-) the HLA-DR antigen, resulted in the selection of Lin-34+ DR+ and Lin-34+ DR- cell populations. When the Lin-34+ DR+ cell fraction was cultured in a short-term methylcellulose assay, we demonstrated a 61-fold enrichment for colony forming cells (CFC) compared with undepleted bone marrow mononuclear cells. In contrast to the Lin-34 …