Prosthetic small-caliber vascular grafts give poorer patency rates than autogenous vein grafts, possibly because the former never spontaneously form endothelium. Animal studies have shown that endothelialization of prosthetic grafts can be encouraged by seeding endothelial cells into the graft at the time of surgery, resulting in improved patency. Information regarding the attachment characteristics of adult human endothelial cells to prosthetic grafts is, however, sparce. Laboratory experiments were performed by use of cell culture techniques to compare the attachment characteristics of adult human endothelial cells to expanded polytetrafluoroethylene graft material, both untreated and treated, with one of four protein substrates--preclotted blood, fibronectin, laminin, and type 4 collagen. Attachment characteristics were compared quantitatively by use of attachment assays and qualitatively by scanning electron microscopy. Attachment to untreated expanded polytetrafluoroethylene was poor but could be greatly improved by preclotting or precoating with any of the proteins, particularly fibronectin. In preclotted grafts seeded endothelial cells formed a virtually confluent monolayer after 1 hour. Cell attachment to the other grafts coated with protein was patchy and inconsistent. It is concluded that a rapid confluent endothelial lining within a prosthetic vascular graft is possible, and of the substrates examined, preclotted blood best encourages cell attachment to expanded polytetrafluoroethylene.