The present invention is directed to a sheath for use with vascular prostheses derived from donor blood vessels, particularly mammalian blood vessels. A vascular prosthesis of the present invention employs an external sheath around a donor blood vessel. The sheath prevents access to the donor vessel wall by host cells originating from perigraft tissue. While resistant to host cell ingrowth, the external sheath is permeable to the flux of macromolecules across its thickness. The exclusion of host cells by the external sheath and the bi-directional flow of macromolecules through the external sheath assists in maintaining the original function of the underlying donor vascular tissue of the prosthesis.