We hypothesized that autologous clot deposited on the luminal surface of laser vascular welds immediately after creation would produce higher time zero bursting pressures than could be achieved in welds perfused with saline alone. To test this hypothesis, we compared bursting pressures of welds created in isolated rabbit aortic segments 1) with saline perfusion only, 2) with blood perfusion, and 3) with blood perfusion followed by infusion of urokinase. Tissue welds with saline perfusion had a mean bursting pressure of 159 2 45 mm Hg; tissue welds following blood perfusion had a mean pressure of 262 f 29 mm Hg; tissue welds with blood perfusion followed by urokinase infusion had a mean bursting pressure of 187 f 35 mm Hg. The saline and urokinase groups were not significantly different. However, the blood-perfused group was significantly higher than both the saline group and the urokinase group. Thus, the addition of urokinase eliminates the beneficial effects noted after blood reperfusion. These observations suggest that the enhancement of weld strength following exposure to blood is due predominantly to the adherence of fibrin-platelet aggregates at the site of the weld.