The demonstration of endothelium lining vascular prostheses in goats1 and baboons, 2 and vascular homografts and prostheses in dogs8 stimulated interest in the source of the endothelium. The most likely assumption was that the host endothelium proliferated and extended from the sutured ends to cover the replacement. The demonstration of a complete endothelial lining in a dacron prosthesis, 4.5 cm. long and in place for 7 days in a growing young pig, 4 cast serious doubt that the sutured ends were the only source of the endothelium. The observation of islands of endothelium appearing in the centers of vascular pros-theses5 supported the concept that the proliferating capillaries that penetrated the interstices of the prosthesis contributed endothelium to the lining. Evidenceis now presented that endothelium can formfrom cells circulating in the blood and independently of anastomotic sites and capillaries, on an intraprosthetic dacron hub isolated within the thoracic aorta of growing young pigs.