The conditions, accuracy, precision and possible error of the thrombin time assay of plasma fibrinogen are determined. Comparison with an estimation of clottable protein by absorbance at 280 nm gave a correlation coefficient of 0.96 and the regression line y = 1.00 x + 0.56 (n = 34). Comparison with a radial immunodiffusion method yielded the correlation coefficient 0.97 and the regression line y = 1.18 x = 2.47 (n = 26). The presence of heparin in clinically applied concentrations produced a slight shortening of the clotting times. The resulting error in the estimated concentrations of fibrinogen was too small to affect the clinical usefulness of the determinations. The influence of fibrin(ogen) degradation products was significant only in excessive amounts in samples containing low levels of fibrinogen.