Coagulation factor XIII (fibrin stabilizing factor, FSF) is detectable in plasma, platelets, placenta and various tissues. In the activated form FSF has the enzymatic properties of a transglutaminase and is capable of stabilizing fibrin by inducing covalent bondings between fibrin monomers. In patients with congenital factor XIII deficiency or acquired immune inhibitors of fibrin stabilization a severe bleeding tendency is evident. There is not yet enough information available concerning the significance of reduced FSF-activity as cofactor in hemorrhagic diathesis and wound healing disturbances in various disease states. There are some indications from experimental studies that there might be an influence of FSF on tumor growth and metastasis as well as arteriosclerosis. The quantitation of the enzyme by radiological and immunological techniques yield reproducible results. Fibrin in its stabilized or non stabilized form can be discriminated in polyacrylamide gel electrophoresis after reduction of fibrin clots.