It is an object to utilize a novel HSCA-3 antigen occurring in immature hemopoietic stem cells and a monoclonal antibody recognizing said antigen in 1) the immunoassay or flow cytometry involving antibody labeling and 2) the isolation and purification of immature hemopoietic stem cells from human bone marrow cells, umbilical code blood or blood enriched in human hemopoietic stem cells by mobilization with G-CSF.It is directed to a novel HSCA-3 antigen occurring in immature hemopoietic stem cells and to a monoclonal antibody or an immunoreactive fragment thereof which recognizes said antigen.