To examine potential mechanisms by which hematopoiesis may be regulated by endothelial cells within the bone marrow (BM) microenvironment, we have devised a technique for the in vitro study of the interaction of human BM microvascular endothelial cells (BMEC) with hematopoietic cells. Microvessels isolated by collagenase digestion of spicules obtained from filtered BM aspirate were plated on gelatin-coated plastic dishes, and colonies of endothelial cells grown from microvessel explants were further purified by Ulex eu-ropaeus lectin affinity separation. BMEC monolayers isolated by this technique grew in typical cobblestone fashion, stained positively with antibody to factor Vill/von Wille-brand factor, and incorporated acetylated LDL. Immunohis-tochemical studies showed that BM microvessels and BMEC monolayers express CD34, PECAM, and thrombospondin. Incubation of resting BMEC with BM mononuclear hematopoietic cells resulted in the selective adhesion of relatively large numbers of CD34⁺ progenitor cells and megakaryocytes. The binding of purified BM-derived CD34⁺ progenitor cells to BMEC was dependent on divalent cations and was partially blocked by antibodies to CD34. IL-1β treatment of BMEC monolayers resulted in an increase of CD34⁺ progenitor cell adhesion by mechanisms independent of CD34 or divalent cations. BMEC exhibit specific affinity for CD34⁺ progenitor cells and megakaryocytes, suggesting that the BM microvasculature may play a role in regulating the trafficking, proliferation, and differentiation of lineage specific hematopoietic elements, and possibly of pluripotent stem cells within the CD34⁺ population.