In order to estimate the effects of sialic acid residues in fibrinogen on the fibrinogen-fibrin conversion by bovine thrombin the Michaelis constant (Km) and maximum velocity (Vmax) were determined. The Km value obtained by the use of intact-fibrinogen was smaller than that of asialo-fibrinogen. This fact suggests that the sialic acid residues affected the formation of the enzyme-substrate complex. It was also found that in comparison with the asialo-fibrinogen, the intact-fibrinogen was significantly influenced in the gel formation time by the ionic strength in the reaction solution.