In the present study three clinical applications of flow-cytometric quantification of CD34-positive cells in the field of blood and marrow transplantation are evaluated. By daily determination of CD34-positive cells in the peripheral blood after mobilization of stem cells with chemotherapy or growth-factors it is possible to set the optimal time for collection of the transplant material. Thereafter the yield of CD34-positive cells within the harvested material is determined. Finally, the efficacy of positive CD34 selection using a biotin-avidin column is evaluated. Initial experience is presented here.