Fibrin is soluble in 5 [image]-urea whereas the normal plasma clot is insoluble. Besides Ca ions, a plasma component called "fibrin-stabilizing factor" (FSF) is needed for the formation of plasma clot. A test is described to measure the FSF activity of plasma. Under the conditions defined, normal human plasma gives a critical FSF value in amounts between 0.1 and 0.2 ml. There is an indication for elevated FSF values under pathologic conditions. FSF activity of serum is greatly reduced as compared with levels in plasma, because FSF is strongly adsorbed to fibrin.