Segments of rat inferior vena cava modified by proteolytic digestion and dialdehyde starch tanning were found to retain their ability to sensitize allogeneic recipients both in the intradermal and intra-aortic positions. Furthermore, these grafts had a very high aneurysm formation (100%) and aneurysm rupture rate (63%), even when used as isografts, suggesting that the modification procedure so damages the structural integrity of the grafts that use in clinical situations may have disastrous consequences. Clinical trials must be undertaken with great caution and a mild, brief course of immunosuppression may be indicated when modified venous allografts are contemplated in potential organ transplant recipients. Furthermore, prosthetic mesh may be required for support whenever modified allogeneic veins are used for arterial substitutes. In the absence of autologous tissue, the bovine xenograft is probably the material of choice for reconstruction or bypass of small vessels.