The ability to enhance the frequency of progenitors in the peripheral circulation has resulted in peripheral blood progenitor cell collections being the preferred procurement method for autologous transplants and an accepted alternative for allografts. In autografts, peripheral blood-derived progenitor cells appear to have an advantage over conventional marrow transplants in the form of earlier engraftment without compromising sustained marrow function. Studies were performed to evaluate the broad class of CD34+ progenitors and their subtypes in preparations of bone marrow and peripheral blood cells. As a result, peripheral blood appears to contain a larger quantity of more mature progenitors, which is the likely reason for a more rapid return of granulocytes and platelets. However, controveries continue to exist as to whether the measurements derived from these studies can be used to predict time to engraftment and subsequent reestablishment of a marrow reserve. Some of these issues have been addressed in pilot studies where allografts using G-CSF-mobilized peripheral blood progenitor cells were compared to conventionally harvested bone marrow and bone marrow obtained from G-CSF-stimulated donors. The time to engraftment of neutrophils and platelets were shortened when G-CSF-mobilized peripheral blood or marrow progenitor cells were used compared to case-matched grafts of steady-state marrow. The significance of the suggested differences between steady-state bone marrow grafts and mobilized peripheral blood cells remains to be determined. However, preparations of cytokine-mobilized peripheral blood progenitor cell preparations are being used with increasing frequency to replace conventional marrow harvests.