Improved biocompatibility by postfixation treatment of aldehyde fixed bovine pericardium. Long-standing release of locally cytotoxic aldehyde concentrations is responsible for lack of spontaneous endothelialization and increased calcification of glutaraldehyde fixed bovine pericardium. Postfixation treatment with amino acids made in vitro endothelialization of bioprosthetic heart valves possible. Such treated pericardium calcified significantly less (13 +/- 4 micrograms/mg dry weight) than did conventionally processed pericardium (114 +/- 25 micrograms/mg) after 63 days of subcutaneous implantation in rats. To test the ability for spontaneous in vivo endothelialization, 5 sheep had 6 mm grafts made from postfixation treated pericardium (PTP) implanted into the carotid artery, compared to PTFE grafts on the contralateral side, which spontaneously endothelialize in animal models. In a pregnant animal, both grafts occluded. All remaining pericardial grafts remained patent, but one additional PTFE graft occluded and another one was stenosed. The area covered with red thrombus was significantly smaller in the PTP grafts (3.05 +/- 3.9%) than in the PTFE grafts 42 +/- 14% (p = 0.0036); TEM and SEM showed endothelial cells growing directly on the PTP, but only on myofibroblasts in PTFE grafts. Postfixation treatment of glutaraldehyde fixed pericardium aids spontaneous endothelialization and decreases tissue calcification. 