Tissue responses in and to a heterograft collagen tube, used as an arterial substitute in 27 patients, demonstrated by biopsies at intervals from three days to 32 months that this graft was inert and became incorporated within a tube of living collagenous host tissue. Within five weeks of implantation, the operative debris was cleared and the host collagenous tissue surrounding the graft was well developed. Incorporation of the graft by the host tissue continues so that at eight months the graft collagen was difficult to identify in the adventitial layer. Atheromatous changes appeared after several months in areas which histologically appeared to be evolution of lesions of small dissections of blood into the media. Four grafts with aneurysms showed atheromatous changes; however, other factors may have been involved in their cause.