Endothelial cells grown on filters developed junctional complexes that reduced diffusional transport and increased electrical resistance over the cell layer. Induction of tissue factor by recombinant interleukin-1 beta led to a highly polarized tissue factor expression on the apical cell surface only. After prolonged growth to allow deposition of matrix, removal of the endothelial cells by collagenase or by 0.1 mol/L NH4OH left behind some cellular material as well as tissue factor, which was only detectable in the upper compartment. A human bladder carcinoma cell line, which does not form tight junctions and expresses tissue factor constitutively, showed essentially no polarity. Endothelial cell secretory compounds like von Willebrand factor, tissue plasminogen activator, and plasminogen activator inhibitor-1 were constitutively released to both sides. The added secretion due to recombinant interleukin-1 beta stimulation of the endothelial cells observed for von Willebrand factor and tissue plasminogen activator was, however, localized to the apical surface. The availability of tissue factor on the luminal surface of endothelial cells, ie, allowing contact with factor VII in the flowing blood, has potentially very significant pathophysiological consequences.