A cell population which is composed of cells bearing the stem cell marker CD34 and which are small in size and have little granulation are obtained by separating low density mononuclear hematopoietic cells according to size and then selecting for CD34.sup.+ cells in the smallest size fraction. The size of the cell population corresponds to that obtained at a flow rate of 25-29 ml/min in a counterflow elutriation method using a rotor equivalent to Beckman JE 5.0 spun at 900.times.g. This population of cells consists essentially of very early progenitor cells and the cells are useful in autologous bone marrow transplantation as well as gene therapy.