A procedure utilizing protamine sulfate to demonstrate the presence of fibrin monomers is described. The procedure is unaffected by heparin and by fibrinolytic split products. Interference by high levels of plasmin, however, was suggested. The possibility of precipitating protein fractions other than fibrin monomers has been lessened by utilizing a low concentration of protamine sulfate (0.9%) and a temperature of 37 C. This method was more sensitive and less subject to interference than two other procedures used to demonstrate fibrin monomers. Its application in differentiating disseminated intravascular coagulation from primary fibrinolysis is discussed.