Thes document is applicable to the calibration of clotting time measurement procedures using calibration plasmas. The calibation can be carried out with the aid of serial dilutions of a defined normal plasma (activity = 100 % or 1 IU/ml) or of a calibration plasma or with the aid of several calibration plasmas with an exactly determined activity or concentration of the coagulant constituent to be analyzed (e.g. fibrinogen in g/1).