The subject invention pertains to antibodies that have binding specificity for an antigen that is expressed on a subset of human, hematopoietic mononuclear cells, including a hematopoietic stem cell population, but is not expressed on normal, mature myeloid cells. In one embodiment, a monoclonal antibody, MG1, is provided. This antibody is useful in methods of isolating cell suspensions from human blood and marrow that can be employed in bone marrow transplantation, genetic therapy, and in treating other diseases of the hematopoietic system. Cell suspensions containing MG1.sup.+ human hematopoietic cells are also provided, as well as therapeutic methods employing the cell suspensions. The subject invention also pertains to the novel antigen recognized by the subject antibodies.