Enzyme-treated bovine arterial heterografts can function as segmental replacements in the abdominal aorta of the dog for long periods of time.1-4 The digestion of the vessel wall with a protease, ficin, and its subsequent formalin tanning, removes almost all of the smooth muscle and elastica, leaving a nonporous, strong collagen tube which does not leak blood through its substance. In a group of 33 dogs, 12 of which were followed for approximately 5 years, the percentage of "successful" abdominal aortic grafts decreased from 73% of 2½ year survivors to 50% of those allowed to live the full duration of the study.4 All long-term grafts classed as "failures" were patent and unruptured at the time of final examination. Of 10 untreated controls with a 10% incidence of "success" by aortography at 2½ years (1 graft), 4 were kept for 5-year evaluation. Three were failures. One was "successful" grossly, but