Objective. Studies of megakaryocytic progenitor cell interactions have focused on single receptor-ligand interactions using isolated components of the extracellular matrix. To approach a physiologic condition, we studied megakaryocytic development of human progenitor cells cultured on two stromal cell lines and on human bone marrow stroma. Materials and Methods. Human CD34+ cells were cocultured with stromal layers in the presence of thrombopoietin. Megakaryocytes were quantified by monoclonal antibodies against glycoprotein (GP) IIb/IIIa (CD41) and GPIX (CD42a). Megakaryocytic clonogenic capacity (burst-forming unit-megakaryocyte and colony-forming unit-megakaryocyte) was determined using fibrin clot assays. Results. After 6 days, a higher percentage of megakaryocytes and more megakaryocytic colonies were recovered from the adherent cell fraction compared to the nonadherent cell fraction. In contrast, significantly more granulocytic and erythroid colonies were recovered from the nonadherent cell fraction. Repeated replating of nonadherent cells onto fresh stroma showed a decline in megakaryocytic recovery of the remaining adherent cells, pointing toward selective adhesion of megakaryocytic progenitors. This was supported further by the finding that burst-forming unit and colony-forming unit megakaryocytes were preferentially recovered from the adherent cell fraction at 24 hours. No effect of blocking the β1 integrins VLA-4 and VLA-5 on human progenitor cells was observed. A higher expression of CD42a antigen and a higher percentage of morphologically recognizable polyploid megakaryocytes were found when cells were grown in noncontact cultures compared to when grown adhered to stroma. Conclusion. In contrast to granulocytic and erythroid progenitors, both very early and more mature megakaryocytic progenitors are preferentially located in the adherent fraction in an in vitro stromal model, leading to inhibition of maturation of megakaryocytes. This suggests that the presence of stroma components in ex vivo expansion cultures, aimed at preservation and expansion of megakaryocytic progenitors, might be a prerequisite. Copyright (C) 2000 International Society for Experimental Hematology.