The initial contact between blood and vascular grafts may be a determinant of the fate of the implants. They behave in different ways depending upon their nature: the processed human umbilical vein and the bovine heterograft just lead to minimal thrombotic retention; the expanded PTFE is even more antithrombogenic; the porous polyesters which are porous textiles, should be totally impregnated with a thrombotic matrix to make the wall impervious. These observations were clearly determined by exposing the grafts to blood, incorporating either labeled platelets or labeled fibrinogen, and they were correlated by SEM examination.