Described are determination procedures for fibrinogen and/or fibrinogen-derivatives by matrix-independent turbidimetry. In the FIFTA called procedure the fibrinogen activity is preferably determined in an undiluted sample by addition of thrombin and/or albumin, as well as polybrene if appropriate, in PBS and determination of the increase in absorbance at 405 nm. In the FIATA called procedure the fibrinogen-concentration and/or the concentration of fibrinogen-derivatives is preferably determined by addition of vancomycin and determination of the increase in turbidity at 405 nm. It is standardized by usage of plasma standards of known fibrinogen-activity and/or fibrinogen-concentration.