Summary. The suitability of counterflow centrifugal elutriation (CCE) for reduction of the number of non-stem cells in autologous G-CSF-mobilized peripheral blood stem cell (PBSC) transplants was investigated. By cell size-monitored CCE, small cells could be rapidly separated from the haemopoietic progenitor cells present in leukapheresis product (LP) samples. The large cell fraction contained an average 86 Ϯ 25% of the CD34 þ cells and 76 Ϯ 20% of the granulocyte-macrophage progenitors (CFU-GM) loaded into the separation chamber, and was depleted of 75 Ϯ 18% of the lymphocytes, 89 Ϯ 7% of the erythrocytes and 98 Ϯ 2% of the platelets (n ¼ 21). Due to the presence of high numbers of large immature myeloid cells, which co-elutriated with progenitor cells, enrichment of CD34 þ cells in the large cell fraction was only modest (average 1 . 8 times). No indication of preferential co-elutriation of primitive stem cells with the small cells was obtained. There was no difference in expression of CD38 or Thy-1 on CD34 þ cells between the two elutriation fractions. Frequencies of cobblestone-areaforming cells (CAFC) week 6, which are considered to represent cells with long-term repopulating ability, were reduced in the small cell fractions as compared to those in the unseparated samples and the large cell fractions. On average, 100% of CAFC week 6 were recovered in the large cell fractions (n ¼ 5). In conclusion erythrocytes, platelets and 40-50% of leucocytes can be depleted from G-CSFmobilized PBSC samples by CCE with an almost complete recovery of both clonogenic and primitive stem cells.