The cellular lining of 4-mm polytetrafluoroethylene (PTFE) arterial bypass grafts of differing porosity implanted into baboons for a period of 3 months was investigated. Endothelial outgrowth onto the less porous (30-mu) PTFE grafts varied from approximately 1 to 3 cm, and 75% of the grafts were totally reendothelialized within 3 months. These luminal cells stained positive for Factor VIII.R.Ag. The endothelial replication in the healed grafts was significantly elevated as compared with that observed in adjacent arteries. By electron microscopy, no breaks were detected in the integrity of the endothelium and no increase in indium activity over the graft was recorded after injection of 111In-labeled platelets. To ensure that the increased cell replication was not related to regrowth, a more porous PTFE graft (60 mu) was implanted which reendothelialized within 2 weeks. Three months after implantation, the replication rate of endothelium in the graft was still significantly increased. The authors concluded that this increased replication represented a desquamating nondenuding injury and that flow characteristics along the graft might be responsible.