Endothelial cells (EC) are a major component of the bone marrow and peripheral vasculature microenvironments and contribute to the regulation of hematopoiesis. Human EC cultured from umbilical vein (HUVEC) and adult aorta (HAEC) were compared to determine differences in levels of the multipotent cytokine, Steel factor (SLF), its receptor (Kit), intercellular adhesion molecule-1 (ICAM-1), and endothelial leukocyte adhesion molecule-1 (ELAM-1) before and after stimulation with human interleukin-1 beta (hIL-1 beta), human tumor necrosis factor-alpha (hTNF-alpha), recombinant human (rh) SLF, phorbol 12-myristate 13-acetate (PMA), or calcium ionophore (A23187). HUVEC expressed four-fold higher basal levels of Kit and three-fold higher basal levels of SLF transcripts than HAEC. In contrast, the basal level of ICAM-1 mRNA was four-fold lower in HUVEC than in HAEC. These differences in expression persisted following activation. All five agonists downregulated Kit mRNA levels by 50 to 80%, but there remained a three-fold higher level of expression in HUVEC compared to HAEC. While SLF mRNA expression was increased four-fold by IL-1 beta or TNF-alpha and 50-fold by PMA or A23187, there was still a two-fold higher level in HUVEC than in HAEC. Similarly, production of cell-associated SLF was induced two-fold above basal level by PMA in HUVEC and HAEC, with HUVEC producing two-fold more than HAEC before and after stimulation. Production of soluble SLF was also increased six-fold in HUVEC and HAEC by PMA, but the HAEC produced slightly more than the HUVEC. Expression of ICAM-1 mRNA was increased 11-fold in activated HUVEC and HAEC, but the induced levels of both ICAM-1 and ELAM-1 mRNA were three-fold lower in HUVEC. The time course of SLF mRNA upregulation and Kit mRNA downregulation paralleled the upregulation of both cytoadhesion molecules. Differences between HUVEC and HAEC may be related to their vascular sources, but also suggest that disparate regulation of SLF, Kit, ICAM-1, and ELAM-1 expression could indicate a predisposition of neonatal EC toward impaired cytokine signal transmission.