Method for preparing a control material which is free of therapeutic inhibitors of coagulation factors from the group of the serine proteases and which is suitable for use in a method for checking the accuracy of methods for quantitative determination of therapeutic inhibitors of coagulation factors from the group of the serine proteases, the method for preparing the control material comprising the steps of: (a) dissolving a defined amount of at least one unspecific serine protease inhibitor selected from the group consisting of benzamidine, phenylmethylsulfonyl fluoride and 4-(2-aminoethyl)benzenesulfonyl fluoride in an aqueous buffer solution, and (b) assigning a first target value for the concentration of a first therapeutic coagulation factor inhibitor, wherein the assignment of the first target value for the concentration of the first therapeutic coagulation factor inhibitor is carried out as follows: i) mixing a first partial amount of the solution prepared in step a) with a defined amount of a first coagulation factor and with a substrate having specificity for said coagulation factor to form a first reaction, ii) measuring the amount of a cleavage product of the substrate in the first reaction, iii) comparing the amount of the cleavage product measured in the first reaction with predetermined calibration values which assign the amount of the cleavage product measured in the first reaction to a known concentration of the first therapeutic coagulation factor inhibitor, and ascertaining the concentration of the first therapeutic coagulation factor inhibitor that is associated with the measured amount of the cleavage product, and iv) defining the thus ascertained concentration of the first therapeutic coagulation factor inhibitor as the first target value of the control material, wherein the first therapeutic coagulation factor inhibitor specifically inhibits the first coagulation factor and wherein the unspecific serine protease inhibitor inhibits the first coagulation factor and at least one further coagulation inhibitor. (a) dissolving a defined amount of at least one unspecific serine protease inhibitor selected from the group consisting of benzamidine, phenylmethylsulfonyl fluoride and 4-(2-aminoethyl)benzenesulfonyl fluoride in an aqueous buffer solution, and (b) assigning a first target value for the concentration of a first therapeutic coagulation factor inhibitor, i) mixing a first partial amount of the solution prepared in step a) with a defined amount of a first coagulation factor and with a substrate having specificity for said coagulation factor to form a first reaction, ii) measuring the amount of a cleavage product of the substrate in the first reaction, iii) comparing the amount of the cleavage product measured in the first reaction with predetermined calibration values which assign the amount of the cleavage product measured in the first reaction to a known concentration of the first therapeutic coagulation factor inhibitor, and ascertaining the concentration of the first therapeutic coagulation factor inhibitor that is associated with the measured amount of the cleavage product, and iv) defining the thus ascertained concentration of the first therapeutic coagulation factor inhibitor as the first target value of the control material,