Bleeding through graft interstices after replacement of the ascending aorta can occur despite the use of low-porosity, woven graft material and standard preclotting methods. To avoid this complication, improved techniques of preclotting by autoclaving the graft after immersion to either blood or plasma have been developed. We used a quantitative method to compare graft permeability in vitro. The control leak rate (LR) was 111 +/- 7 ml/cm2/min. Standard preclotting reduced this to 23 +/- 4 ml/cm2/min (p less than 0.01). Grafts were autoclaved after immersion in heparinized whole blood (LR 3 +/- 4 ml/cm2/min), 5% albumin (LR 5 +/- 2 ml/cm2/min), plasma alone (LR 4 +/- 0.3 ml/cm2/min) and plasma with thrombin added (LR 2 +/- 0.2 ml/cm2/min). All autoclaved grafts were less permeable than those preclotted by the standard method (p less than 0.01). Scanning electron microscopy showed that the interstices of autoclaved grafts were more uniformly filled than those of grafts prepared by standard preclotting. Perfusion with fibrinolysins significantly (p less than 0.01) increased leak rates of grafts with standard preclotting but did not affect autoclaved grafts. We conclude that autoclaving grafts after immersion in blood, plasma or albumin decreases graft permeability and that autoclaved grafts are resistant to fibrinolysis.