Turbidimetric specific techniques (turbidity end-pot measurement after thrombin addition to the plasma) are widely used for fibrinogen determination. This paper describes a series of tests performed with the aim of establishing a fixed-time kinetic method based on the above mentioned technique. It is know that 1st order or pseudo 1st order reaction are the most valuable for the kinetic determination of substrates. However for this procedures, in contrast to end-point techniques, enzymes with the highest possible Michaelis constant are required. If the Michaelis constant values for the pair fibrinogen/thrombin determined in artificial systems have a molarity in the order of 10(-5)/l when the measurement is made on plasma, antithrombins which are powerful thrombin inhibitors, increase the Michaelis constant to a molarity of 10(-2)/l. For the thrombin in the reaction mixture we have adopted an activity of 1.6 National Institute of Health (NIH) units/100 microliter of plasma; this activity is sufficiently high to induce instantaneous start of fibrinogen polymerization, without affecting the 1st order kinetic. The preliminary studies which have been carried out to evaluate this technique have shown the following characteristics: a) it is both accurate and precise; b) it requires simple and fast operations; c) it may easily be automated with a productivity of about 200 tests/hour on micro centrifugal analyzer.