Abstract A method, independent of cloning, and using the highly specific precipitant of fibrinogen, potassium mercuric thiocyanate [K 2Hg(CNS) 4], is described for determining the fibrinogen content of human plasmas that clot normally upon the addition of thrombin or of human plasmas with added heparin or Pyran (divinyl ether-maleic acid copolymer) that do not clot or clot abnormally when thrombin is added. The plasma is mixed with phosphate buffer, pH 7.4, 6-aminocaproic acid, NaCl, and K 2Hg(CNS) 4 and the protein content of the precipitated fibrinogen is determined. After washing and dissolving, the fibrinogen precipitated by the thiocyanate salt clots normally when thrombin is added. However, if either haparin or Pyran is present during the precipitation step, normal clots do not form when thrombin is added. These data confirm the results of other investigators who showed that the thiocyanate reagent does not change the clottability of fibrinogen and that both heparin and Pyran change fibrinogen so that normal clots do not form.