Human umbilical cord vein allografts (HUCVAG) were prepared by sequential treatment of fresh cords with 95% ethanol and 1.3% dialdehyde starch. Of 30 canine aortic implants of fully treated grafts, 26 were patent at the end of 8 mo. Among 25 aortic interpositions of untreated and 70% ethanol-treated grafts, the mean survival was 13 and 34 days, respectively. Five of ten 70% ethanol:1.3% dialdehyde starch-treated implants remained patent for 8 mo. Graft failure was attributed to incomplete antigenic suppression manifested by rejection changes, including thrombosis, anastomotic disruption, aneurysm formation and intimal degeneration. HUCVAG has been used in 5 patients who required femoropopliteal bypass. Of 3 successful revascularizations, 2 remain patent beyond 8 mo. without aneurysm formation. The processed, valveless, unbranched HUCVAG, in unlimited supply, may serve as an available alternative in the absence of a suitable saphenous vein.