Peripheral blood stems cells (PBSCs) have been used in autologous transplantation as an alternative to bone marrow-derived cells. Recently, PBSCs have been collected from healthy donors after priming with G-CSF and used for allogeneic transplantation. We have a comparatively large experience with PBSC collection in autologous and allogeneic settings. The five cell separators we employ are: the CS3000 plus, AS 104, Excel, Cobe Spectra and MCS 3p. These machines appear to have different efficacies but no studies have been carried out on this topic. In a prospective study we have randomly assigned donors to different cell separators to evaluate their efficiency. Twenty-five donors underwent the procedure and 50 leukaphereses were carried out. Donors were given 5 micrograms kg-1 d-1 of recombinant human G-CSF for 3 days and 10 micrograms kg-1 d-1 for 4 days subcutaneously. Leukaphereses were performed on days 6 and 7 of G-CSF administration. The results of our study show that a total value of CD34+ cells ranging from 48.44 x 10(6) to 270.37 x 10(6) can be collected from donors with a white cell count ranging from 40.50 x 10(3) microL-1 to 51.34 x 10(3) microL-1 and mononuclear cells ranging from 16.42 to 20.37%. The Excel and the MCS 3p seem to differ from the other machines in terms of higher CD34+ cell collection efficiency. The Excel appears to be even more efficient than the MCS 3p, but this may not reflect reality because the Excel processes 12 L of blood while the MCS 3p employs an 8 L procedure. All the machines showed satisfactory results in terms of yield and quality of the harvests.