Factor XIII (FXIII) promotes fibrin crosslinking and red blood cell (RBC) retention in clots. The FXIII-A polymorphism, Val34Leu, is associated with protection against venous thrombosis. This effect is hypothesized to result from fibrinogen concentration-dependent changes in fibrin structure. Effects of the FXIII-A Val34Leu polymorphism in whole blood clots have not been investigated.