The purpose of this study was to compare the ability to collect human bone marrow-derived mesenchymal (stromal) progenitor cells (MPC) from bone marrow versus peripheral blood hematopoietic progenitor cell (PBPC) collections using in vitro and in vivo assays. Ten milliliter samples of PBPC collections mobilized from 11 patients undergoing autotransplants using chemotherapy followed by G-CSF 5-10 micrograms/kg were evaluated using in vitro and in vivo assays for hematopoietic progenitors and MPCs. Additionally, 10 ml samples of unstimulated bone marrow aspirates as well as PBPC collected after mobilization using G-CSF 10 micrograms/kg obtained from 3 normal, histocompatible allogeneic donors were analyzed for hematopoietic progenitors and MPCs. The MPCs were isolated and culture-expanded as adherent cells in vitro and subsequently tested for the capacity to differentiate into mesenchymal phenotypes in vivo using calcium hydroxyapatite porous ceramic cubes implanted s.c. in athymic mice. Demineralized sections of these cubes were analyzed histologically for the appearance of bone and cartilage. Seven autotransplant subjects with cancer received G-CSF after chemotherapy administration, whereas 4 cancer patients and all 3 normal donors received G-CSF alone as the mobilizing regimen. For the autologous PBPC collections and the normal marrow aspirations, median hematopoietic progenitor content was in the normal range for our institution. MPCs were detected in in vitro cultures and as bone-positive ceramic cubes in samples of all 3 allogeneic donor bone marrows but in none of the 14 autologous and 6 allogeneic PBPC collections. In conclusion, MPCs could not be recovered in PBPC collections obtained from either normal donors or patients who underwent PBPC collections after mobilization therapy but could be obtained routinely from bone marrow samples. Although the role of transplanted MPCs is an area of clinical investigation, this study points out a fundamental differences in the population of cells transplanted after collection from bone marrow versus peripheral blood.