A recently described long-term culture system for early human progenitor cells was established with the murine preadipocyte stromal line FBMD-1 grown in 96-well plates; cobblestone areas formed by inoculated hematopoietic cells are determined in a limiting dilution setting after five weeks' culture. To compare the capacity of cobblestone-area-forming cell (CAFC) formation by bone marrow and leukapheresis products in this system, mononuclear cells (MNC) of both origins were cultured. As related to CD34+ cell content, CAFC yields after five weeks' culture were in the same range in bone marrow and leukapheresis stemming from patients with efficient mobilization of hematopoietic cells. In purified CD34+ cell fractions, the CAFC yield per inoculated cell number was considerably higher than in MNC; however, if the CAFC number was related to the inoculated CD34+ cell number in MNC and after purification, the yield was four to eight times decreased in purified fractions. Addition of the mature cells brought the CAFC yield back up to the numbers obtained in the unseparated MNC fraction. By contrast, slightly more advanced progenitors per CAFC were found in cultures of purified hematopoietic cells from both origins than in whole MNC. The results suggest that mature human accessory cells give noticeable support to recruitment of early progenitors on this feeder but lead to lower yield of GM progenitors.