Objective. Coculture with stromal cells enhances preservation and self-renewal of primitive progenitor potential in hematopoietic cells during ex vivo culture with growth factors (GF). However, the respective roles of growth factors, stromal contact, and extracellular matrix (ECM) ligands in this effect are not clear. Here we investigated the role of direct contact with stroma and the ECM protein fibronectin (FN) in these effects, and investigated whether abnormal integrin receptor function in chronic myelogenous leukemia (CML) progenitors was associated with perturbation in these responses. Methods. Normal bone marrow CD34+ cells were cultured in GF-containing medium with or without contact with stromal layers, glutaraldehyde-fixed stromal layers (stroma-contact), or integrin-binding FN fragments for 7 days. Progeny cells were assayed for primitive progenitors in week-6 long-term culture-initiating cell (LTC-IC) and week-10 extended LTC-IC (ELTC-IC) assays. Results. Increased LTC-IC and ELTC-IC preservation was seen following coculture with stroma, and was also observed after culture in contact with fixed stromal layers and FN. Both α4β1 and α5β1-integrin binding FN fragments enhanced LTC-IC preservation. Analysis of single CD34+CD38- cells showed that coculture with FN resulted in significantly reduced cell division, but enhanced retention of LTC-IC capacity in divided cells. FN also increased LTC-IC frequency in undivided cells. CML progenitors demonstrate deficient integrin-mediated adhesion, migration, and signaling. Coculture of CML CD34+ cells with stroma and FN failed to enhance LTC-IC preservation. Conclusion. We conclude that β1 integrin-FN interactions enhance normal primitive progenitor preservation with or without cell division, and that these mechanisms are impaired in CML primitive progenitors. © 2002 International Society for Experimental Hematology. Published by Elsevier Science Inc.