Laboratory and clinical evaluation of a knitted Dacron graft impregnated with gelatin to confer zero porosity is described. Graft performance was tested by standard methods for biodegradation of the sealant and in vitro thrombogenicity. The gelatin sealant was removed after seven to nine days and there was no platelet adhesion to Gelseal compared with unsealed Dacron. Animal experiments revealed normal macroscopic appearances in the graft and histological disappearance of the gelatin impregnate between five and ten days, allowing a cellular response similar to unsealed Dacron. The first 100 patients to have Gelseal aortic bifurcation graft implanted at Glasgow Royal Infirmary are described. The graft did not require preclotting. Blood transfusion was not necessary in 74% of patients. There is 100% patency at 21 months. A knitted Dacron graft sealed with gelatin is a safe, nonporous prosthesis at implantation.