A new closed system for counterflow centrifugation (elutriation) is described. The system was developed to increase the capacity of counterflow centrifugation in order to be able to separate bone marrow intended for allogeneic bone marrow transplantation within 3 h. The rotor has the capacity for up to four separation chambers, offering the possibility of separating either a single-cell suspension under equal or differing conditions, or four different suspensions simultaneously. Profiles of low-density nucleated cells from normal blood were shown to be identical after elutriation in four different chambers. Leucocytes could be depleted from platelet concentrates without significant loss. Most (98%) of the lymphocytes were removed from donor marrow intended for transplantation within 3 h and the recovery of myeloid and erythroid clonogenic cells in the graft was similar to that obtained from the standard single chamber centrifuge.