SummaryA Quantitative, simple method for fibrinogen determination is described. Results are obtained within three minutes.The quantitative interaction of thrombin and fibrinogen is the basic principle of the test. Different factors, which could interfere in the reaction, are critically evaluated viz. :—thrombin concentration;—accelerators of fibrin formation (calcium ions, plasma proteins, fibrino-plastic substances);—inhibitors (heparin, antitlnombin V and VI);—non specific influences such as ionic strength, pH and temperature;—red cells.The results lead to a preliminary technique, which determines the clottable fibrinogen independently of the factors mentioned.