We studied the change in water porosity over time of 10 Sauvage Bionit-II and 10 DeBakey Vasculour-II knitted velour Dacron grafts throughout the four stages of the Sauvage preclotting technique. Graft porosity decreased significantly (p less than 0.001) at the ends of stages 1 and 2 for both types of grafts, but stages 3 and 4 did not further reduce graft porosity. These results demonstrate that a two-stage preclotting technique is adequate for the preclotting of knitted velour Dacron grafts. A final rinse with heparinized blood is recommended as this may reduce graft thrombogenicity.