Since surgical endarterectomy produces a highly thrombogenic vascular flow surface, we evaluated in baboons the capacity of acutely attached, unspread, cultured, homologous aortic endothelial cells to form durable, confluent endothelial cell monolayers and to interrupt acute thrombus formation during exposure to arterial blood flow conditions. When cultured baboon aortic endothelial cells were incubated with segments of freshly endarterectomized baboon aorta for 20 minutes in vitro, 3.8 x 10(5) endothelial cells/cm2 became attached. The endarterectomized surface treated with endothelial cells showed the presence of rounded, unspread endothelial cells with intervening bare areas of vascular media. Platelet deposition onto the endothelial cell--treated surface during exposure to blood under high flow conditions for 1 hour in vivo was markedly reduced by the attached but unspread endothelial cells. Moreover, these attached endothelial cells underwent spreading on the endarterectomized surface in the presence of high shear blood flow to form a confluent endothelial monolayer at the end of 1 hour. Thus acutely attached cultured endothelial cells form a durable thromboresistant monolayer on surgically denuded vascular surfaces during exposure to arterial blood flow.