Endothelial seeding of vascular grafts may reduce thrombogenicity and significantly improve graft patency. For clinical studies endothelial cells derived from human omentum may be the ideal source of cells as they are obtainable in large numbers. This study was designed to assess their ability to produce the antithrombogenic substance prostacyclin (PGI 2). Human omental microvascular endothelial cells (HOTMECs) were grown and their ability to produce prostacyclin (PGI 2), PGE 2, PGF 2α and thromboxane A 2 (TXA 2) in response to a range of agonists was measured by radioimmunoassay. Control experiments were performed using human umbilical vein endothelial cells (HUVECs). Both HUVECs and HOTMECs synthesised similar quantities of PGI 2 basally and both increased production after adding A23187 (calcium ionophore), arachidonic acid, sodium fluoride and phorbol ester. In contrast, both thrombin and histamine were potent stimulators of PGI 2 release in HUVECs but were without effect on HOTMECs. In both cell types serotonin, carbachol and noradrenaline were without effect. The pattern of release of PGE 2, PGF 2α, and TXA 2 were identical to those of PGI 2 in both cell types but the quantities released were lower. These results show that HOTMECs can produce the antithrombogenic agent PGI 2 in significant quantities and that they do so by mechanisms similar to those of large vessel endothelium. This supports the proposal that they would be a suitable cell source for vascular graft seeding.